Unless the context requires otherwise, all references to the "Company," "SL Green," "we," "our" and "us" in this annual report means SL Green Realty Corp., a Maryland corporation, and one or more of its subsidiaries, including the Operating Partnership, or, as the context may require, SL Green only or the Operating Partnership only, and "S.L. Green Properties" means S.L. Green Properties, Inc., a New York corporation, as well as the affiliated partnerships and other entities through which Stephen L. Green historically conducted commercial real estate activities.

In connection with the Company's initial public offering, or IPO, in August 1997, the Operating Partnership received a contribution of interests in real estate properties as well as a 95% economic, non-voting interest in the management, leasing and construction companies affiliated with S.L. Green Properties. We refer to these management, leasing and construction entities, which are owned by S.L. Green Management Corp, as the "Service Corporation." The Company is organized so as to qualify, and has elected to qualify as a REIT, under the Internal Revenue Code of 1986, as amended, or the Code.

Substantially all of our assets are held by, and all of our operations are conducted through, the Operating Partnership. We are the sole managing general partner of the Operating Partnership, and as of December 31, 2017,2019, we owned 95.42%94.97% of its economic interests. All of the management and leasing operations with respect to our wholly-owned properties are conducted through SL Green Management LLC, or Management LLC. The Operating Partnership owns a 100% interest inof Management LLC.

In order to maintain the Company's qualification as a REIT while realizing income from management, leasing and construction contracts with third parties and joint venture properties, all of these service operations are conducted through the Service Corporation, a consolidated variable interest entity. We, through our Operating Partnership, receive substantially all of the cash flow from the Service Corporation's operations. All of the voting common stock of the Service Corporation is held by an entity owned and controlled by Stephen L. Green, who serves as a member and as the chairman emeritus of the Company's board of directors.

SL Green is New York City'sManhattan's largest owner of commercialoffice real estate and an investment-grade, S&P 500 company that is focused primarily on owning, managing and maximizing the value of Manhattan commercial properties.

Our core business is the ownership of high quality commercial properties and our primary business objective is to maximize the total return to stockholders, through growth in net income attributable to common stockholders and funds from operations, or FFO, and through asset value appreciation. The commercial real estate expertise resulting from owning, operating, investing, developing, redeveloping and lending on real estate in Manhattan for over 3739 years has enabled us to invest in a collection of premier office and retail properties, selected multifamily residential assets, and high quality debt and preferred equity investments. We also own high quality officecommercial properties in the New York metropolitan area.

We are led by a strong, experienced management team that provides a foundation of skills in all aspects of real estate, including acquisitions, dispositions, management, leasing, development, redevelopment, and financing. It is with this team that we have achieved a market leading position in our targeted submarkets.

We seek to enhance the value of our company by executing strategies that include the following:

We seek to capitalize on our management's extensive knowledge of Manhattan and the New York City and surrounding suburban marketsmetropolitan area and the needs of our tenants through proactive leasing and management programs, which include: (i) use of in-depth market experience resulting from managing and leasing tens of millions of square feet of office, retail and residential space since the Company was founded, predominantly in Manhattan;founded; (ii) careful tenant management, which results in long average lease terms and a manageable lease expiration schedule; (iii) utilization of an extensive network of third-party brokers to supplement our in-house leasing team; (iv) use of comprehensive building management analysis and planning; and (v) a commitment to tenant satisfaction by providing high quality tenant services at competitive rental rates.

We acquire core properties for long-term value appreciation and earnings growth. We also acquire non-core properties that are typically held for shorter periods during which we intend to create significant increases in value. This strategy has resulted in capital gains that increase our investment capital base. In implementing this strategy, we continually evaluate potential acquisition opportunities. These opportunities may come from new properties as well as acquisitions in which we already hold a joint venture interest or, from time to time, from our debt and preferred equity investments.

Through intimate knowledge of our markets we have developed an ability to source transactions with superior risk-adjusted returns by capturing off-market opportunities. In rising markets, we primarily seek to acquire strategic vacancies that provide the opportunity to take advantage of our exceptional leasing and repositioning capabilities to increase cash flow and property value. In stable or falling markets, we primarily target assets featuring credit tenancies with fully escalated in-place rents to provide cash flow stability near-term and the opportunity for increases over time.

We continually evaluate our portfolio to identify those properties that are most likely to meet our long-term earnings and cash flow growth objectives and contribute to increasing portfolio value. Properties that no longer meet our objectives are evaluated for sale, or in certain cases, joint venture to release equity created through management's value enhancement programs or to take advantage of attractive market valuations.

We seek to efficiently deploy the capital proceeds generated from these dispositions into property acquisitions and debt and preferred equity investments that we expect will provide enhanced future capital gains and earnings growth opportunities. Management may also elect to utilize the capital proceeds from these dispositions to repurchase shares of our common stock, repay existing indebtedness of the Company or its subsidiaries, repurchase shares of our common stock, or increase cash liquidity.

Our extensive knowledge of the markets in which we operate and our ability to efficiently plan and execute capital projects provide the expertise to enhance returns by repositioning properties that are underperforming. Many of the properties we own or seek to acquire feature unique architectural design elements including large floor plates, andor other amenities and characteristics that can be appealing to tenants when fully exploited. Our strategic investment in these properties, combined with our active management and pro-active leasing, provide the opportunity to creatively meet market needs and generate favorable returns.

Our constant interactions with tenants and other market participants keep us abreast of innovations in workplace layout, store design and smart living. We leverage this information to identify properties primed for development or redevelopment to meet these demands and unlock value. Our deep visibility into the market allows us to acquire locations before others see the opportunity. The expertise and relationships that we have built from managing complex construction projects in New York City

We invest in well-collateralized debt and preferred equity investments in the markets in which we operate, primarily New York City, that generate attractive yields. See Note 5, "Debt and Preferred Equity Investments," in the accompanying consolidated financial statements. Knowledge of our markets and our leasing and asset management expertise provide underwriting capabilities that enable a highly educated assessment of risk and return. The benefits of this investment program, which has a carefully managed aggregate size, net of anticipated payoffs, generally not to exceed 10% of our total enterprise value, include the following:

Our objective is to maintain multiple sources of corporate and property level capital to obtain the most appropriate and lowest cost of capital. This objective is supported by:

Manhattan is by far the largest office market in the United States containing more rentable square feet than the next four largest central business district office markets combined. The properties in our portfolio are primarily concentrated in some of Manhattan's most prominent midtown locations.

According to Cushman and Wakefield Research Services as of December 31, 2017,2019, Manhattan has a total office inventory of approximately 399.0408.9 million square feet, including approximately 243.1250.7 million square feet in midtown. We estimate that approximately 18.313.6 million square feet of new construction class-A buildings over 250,000 square feet will become available between 20182020 and 20222024 in Manhattan, approximately 45.4%43.7% of which is pre-leased. We estimate that this increase is partially offset by approximately 4.45.5 million square feet which will be converted from office use to an alternative use. This will add only approximately 0.7%0.4% per year to Manhattan's total inventory, net of conversions, over the next five years.

While the addition of new supply to the Manhattan office inventory is nominal relative to the size of the overall market, we view any additional supply as a positive to the Manhattan office market given the older vintage of the majority of Manhattan’s office inventory and the desire of certain tenants to occupy new, high quality, efficient office space, which often isn’t available in older vintage properties. In addition, Manhattan’s office inventory has only grown by approximately 8.38.6 million square feet over the last 25-3025 years.

Leases entered into for space in Manhattan typically contain terms that may not be contained in leases in other U.S. office markets. The initial term of leases entered into for space in Manhattan is generally seven to fifteen years. Tenants leasing space in excess of 10,000 square feet for an initial term of 10 years or longer often will negotiate an option to extend the term of the lease for one or two renewal periods, typically for a term of five years each. The base rent during the initial term often will provide for agreed-upon periodic increases over the term of the lease. Base rent for renewal terms is most often based upon the then fair

In addition to base rent, a tenant will generally also pay its pro rata share of increases in real estate taxes and operating expenses for the building over a base year, which is typically the year during which the term of the lease commences, based upon the tenant's proportionate occupancy of the building. In some smaller leases (generally less than 10,000 square feet), in lieu of paying additional rent based upon increases in building operating expenses, base rent will be increased each year during the lease term by a set percentage on a compounding basis (though the tenant will still pay its pro rata share of increases in real estate taxes over a base year).

Tenants typically receive a free rent period following commencement of the lease term, which in some cases may coincide with the tenant's construction period.

The landlord most often supplies electricity either on a sub-metered basis at the landlord's cost plus a fixed percentage or on a rent inclusion basis (i.e., a fixed fee is added to the base rent for electricity, which amount may increase based upon increases in electricity rates or increases in electrical usage by the tenant). Base building services, other than electricity, such as heat, air conditioning, freight elevator service during business hours and base building cleaning typically are provided at no additional cost, but are included in the building's operating expenses. The tenant will typically pay additional rent only for services which exceed base building services or for services which are provided other than during normal business hours.

In a typical lease for a new tenant renting in excess of 10,000 square feet, the landlord will deliver the premises with existing improvements demolished. In such instances, the landlord will typically provide a tenant improvement allowance, which is a fixed sum that the landlord makes available to the tenant to reimburse the tenant for all or a portion of the tenant's initial construction of its premises. Such sum typically is payable as work progresses, upon submission by the tenant of invoices for the cost of construction and lien waivers. However, in certain leases (most often for relatively small amounts of space), the landlord will construct the premises for the tenant at a cost to the landlord not to exceed an agreed upon amount with the tenant paying any amount in excess of the agreed upon amount. In addition, landlords may rent space to a tenant that is "pre-built" (i.e., space that was constructed by the landlord in advance of lease signing and is ready to for the tenant to move in with the tenant selecting paint and carpet colors).

The following table sets forth the weighted average occupancy rates at our office properties based on space leased for properties owned by us as of December 31, 2017 and 2016:2019:

We are constantly evaluating our schedule of future lease expirations to mitigate occupancy risk while maximizing rent growth. We proactively manage future lease expirations based on our view of estimated current and future market conditions and asking rents. The following table sets forth our future lease expirations, excluding triple net leases, and management's estimates of market asking rents. Taking rents are typically lower than asking rents and may vary from building to building. There can be no assurances that our estimates of market rents are accurate or that market rents currently prevailing will not erode or outperform in the future.

The Company is a REIT that is engaged in the acquisition, development, ownership, management and operation of commercial and residential real estate properties, principally office properties, located in the New York metropolitan area and has two reportable segments: real estate and debt and preferred equity investments. Our industry segments are discussed in Note 21, "Segment Information," in the accompanying consolidated financial statements.

At December 31, 2017,2019, our real estate portfolio was primarily located in one geographical market, the New York metropolitan area. The primary sources of revenue are generated from tenant rents and escalations and reimbursement revenue. Real estate property operating expenses consist primarily of cleaning, security, maintenance, utility costs, real estate taxes and, at certain properties, ground rent expense. As of December 31, 2017,2019, one tenant in our office portfolio, Credit Suisse, contributed 8.0%8.2% of our share of annualized

At December 31, 2017,2019, we held debt and preferred equity investments with a book value of $2.3$1.7 billion, including $0.2$0.1 billion of debt and preferred equity investments and other financing receivables that are included in balance sheet line items other than the Debt and Preferred Equity Investments line item. At December 31, 2017,2019, the assets underlying our debt and preferred equity investments were located in the New York metropolitan area. The primary sources of revenue are generated from interest and fee income.

At December 31, 2017,2019, we employed 1,0651,033 employees, 212290 of whom were managers and professionals, 754 of whom were hourly-paid employees involvedemployed in building operations and 99 of whom were clerical, data processing and other administrative employees.our corporate offices. There are currently six collective bargaining agreements which cover the workforce that services substantially all of our properties.

The majority of our property holdings are comprised of commercial office properties located in midtown Manhattan. Our property holdings also include a number ofsome retail properties and multifamily residential properties. As a result of the concentration of our holdings, our business is dependent on the condition of the New York Citymetropolitan area economy in general and the market for office space in midtown Manhattan in particular. Future weakness and uncertainty in the New York Citymetropolitan area economy could materially reduce the value of our real estate portfolio and our rental revenues, and thus adversely affect our cash flow and our ability to service current debt and to pay dividends and distributions to security holders. Similarly, future weakness and uncertainty in our suburban markets could adversely affect our cash flow and our ability to service current debt and to pay dividends and distributions to security holders.

If tenants decide not to renew their leases upon expiration, we may not be able to relet the space. Even if tenants do renew or we can relet the space, the terms of a renewal or new lease, taking into account among other things, the cost of improvements to the property and leasing commissions, may be less favorable than the terms in the expired leases. As of December 31, 2017,2019, approximately 50.7%46.3% and approximately 17.0%26.7% of the rentable square feet, are scheduled to expire by December 31, 20222024 at our consolidated properties and unconsolidated joint venture properties, respectively, and as of December 31, 2017,2019, these leases had annualized escalated rent totaling $514.2$408.6 million and $131.1$268.3 million, respectively. In addition, changes in space utilization by tenants may impact our ability to renew or relet space without the need to incur substantial costs in renovating or redesigning the internal configuration of the relevant property. If we are unable to promptly renew the leases or relet the space at similar rates or if we incur substantial costs in renewing or reletting the space, our cash flow and ability to service debt obligations and pay dividends and distributions to security holders could be adversely affected.

The leasing of real estate is highly competitive. The principal competitive factors are rent, location, lease term, lease concessions, services provided and the nature and condition of the property to be leased. We directly compete with all owners, developers and operators of similar space in the areas in which our properties are located.

Our commercial office properties are concentrated in highly developed areas of midtown Manhattan and certain Suburban central business districts, or CBDs.the New York metropolitan area. Manhattan is the largest office market in the United States. The number of competitive office properties in Manhattan and CBDs in which our Suburban properties are located,the New York metropolitan area, which may be newer or better located than our properties, could have a material adverse effect on our ability to lease office space at our properties, and on the effective rents we are able to charge.

Our interests in 420 Lexington Avenue, 461 Fifth Avenue, 711 Third Avenue, 625 Madison Avenue, 1185 Avenue of the Americas, 1080 Amsterdam Avenue, and 30 East 40th Street, all in Manhattan, and 1055 Washington Avenue, Stamford, Connecticut,certain properties are entirely or partially comprised of either long-term leasehold or operating sublease interests in the land and the improvements, rather than by ownership of fee interest in the land.

Five of our properties, 11 Madison Avenue, 1185 Avenue of the Americas, 1515 Broadway,11 Madison Avenue, 420 Lexington Avenue, 1515 Broadway, and 1One Madison Avenue accounted for 32.8%33.7% of our Portfolio annualized cash rent, which includes our share of joint venture annualized cash rent, as of December 31, 2017.2019.

Our revenue and cash available to service debt obligations and for distribution to our stockholders would be materially adversely affected if any of these properties were materially damaged or destroyed. Additionally, our revenue and cash available to service debt obligations and for distribution to our stockholders would be materially adversely affected if tenants at these properties fail to timely make rental payments due to adverse financial conditions or otherwise, default under their leases or file for bankruptcy or become insolvent.

Giving effect to leases in effect as of December 31, 20172019 for consolidated properties and unconsolidated joint venture properties, as of that date, our five largest tenants, based on annualized cash rent, accounted for 18.0%19.0% of our share of Portfolio annualized cash rent, with two tenants,one tenant, Credit Suisse Securities (USA) LLC, and Viacom International Inc. accounting for 8.0%, and 4.4%8.2% of our share of Portfolio annualized cash rent, respectively. In January 2020, Credit Suisse entered into a lease termination agreement with the Company and vacated its space at the property, thereby reducing its share of annualized cash rent to approximately 3.3%. Our business and results of operations would be adversely affected if any of our major tenants became insolvent, declared bankruptcy, or otherwise refused to pay rent in a timely fashion or at all. In addition, if business conditions in the industries in which our tenants are concentrated deteriorate, or economic volatility has a disproportionate impact on our clients, we may experience increases in past due accounts, defaults, lower occupancy and reduced effective rents across tenants in such industries, which could in turn have an adverse effect on our business and results of operations.

The Company continues its significant ground-upCompany's development project, One Vanderbilt. Construction of the project will not be completed for several years. As with any ground-up development project, unforeseen delaysprojects are subject to internal and otherexternal factors which may affect construction progress. Unforeseen matters could further delay completion, result in increased costs or otherwise have a material effect on our results of operations. In addition, the extended time frame to complete the project willthese projects could cause the projectthem to be subject to shifts and trends in the real estate market which may not be consistent with our current business plans for this property.the properties.

Our business may be affected by volatility in the financial and credit markets and other market, economic, or political challenges experienced by the U.S. economy or the real estate industry as a whole, including changes in law and policy accompanying the new administration and uncertainty in connection with any such changes. Future periods of economic weakness or volatility could result in reduced access to credit and/or wider credit spreads. Economic or political uncertainty, including concern about growth and the stability of the markets generally and changes in the federal interest rates, may lead many lenders and institutional investors to reduce and, in some cases, cease to provide funding to borrowers, which could adversely affect our liquidity and financial condition, and the liquidity and financial condition of our tenants. Specifically, our business may be affected by the following conditions:

Some of the tenants in our properties are smaller, growth-oriented businesses that may not have the financial strength of larger corporate tenants. Smaller companies generally experience a higher rate of failure than larger businesses. Growth-oriented

We earn a significant portion of our income from renting our properties. Our operating costs, however, do not necessarily fluctuate in direct proportion to changes in our rental revenue. As a result, our costs will not necessarilyIf revenues decline even if our revenues do. In such event,more than expenses, we may be forced to borrow to cover our costs, we may incur losses or we may not have cash available to service our debt and to pay dividends and distributions to security holders.

We may acquire properties when we are presented with attractive opportunities. We may face competition for acquisition opportunities from other investors, particularly those investors who are willing to incur more leverage, and this competition may adversely affect us by subjecting us to the following risks:

If we are unable to successfully acquire additional properties, our ability to grow our business could be adversely affected. In addition, increases in the cost of acquisition opportunities could adversely affect our results of operations.

Our business strategy relies upon the acquisition of interests in properties, individual properties and portfolios of properties, including large portfolios that could significantly increase our size and alter our capital structure. Our acquisition activities may not be successful if we are unable to meet required closing conditions or unable to finance acquisitions and developments of properties on favorable terms or at all. Additionally, we have less visibility into the future performance of acquired properties than properties that we have owned for a period of time, and therefore, recently acquired properties may not be as profitable as our existing portfolio.

Further, we may acquire properties subject to both known and unknown liabilities and without any recourse, or with only limited recourse to the seller. As a result, if a liability were asserted against us arising from our ownership of those properties, we might have to pay substantial sums to settle it, which could adversely affect our cash flow. Unknown liabilities with respect to properties acquired might include:

In connection with past and future acquisitions of interests in properties, we have or may agree to restrictions on our ability to sell or refinance the acquired properties for certain periods. These limitations could result in us holding properties which we would otherwise sell, or prevent us from paying down or refinancing existing indebtedness, any of which may have adverse consequences on our business and result in a material adverse effect on our financial condition and results of operations.

We maintain “all-risk” property and rental value coverage (including coverage regarding the perils of flood, earthquake and terrorism, excluding nuclear, biological, chemical, and radiological terrorism ("NBCR")), within three property insurance programs and liability insurance. Separate property and liability coverage may be purchased on a stand-alone basis for certain assets, such as the development of One Vanderbilt. Additionally, one of our captive insurance company,companies, Belmont Insurance Company, or Belmont, provides coverage for NBCR terrorist acts above a specified trigger, although iftrigger. Belmont's retention is reinsured by our other captive insurance company, Ticonderoga Insurance Company ("Ticonderoga"). If Belmont isor Ticonderoga are required to pay a claim under our insurance policies, we would ultimately record the loss to the extent of Belmont's required payment.payments. However, there is no assurance that in the future we will be able to procure coverage at a reasonable cost. Further, if we experience losses that are uninsured or that exceed policy limits, we could lose the capital invested in the damaged properties as well as the anticipated future cash flows from those properties. Additionally, our debt instruments contain customary covenants requiring us to maintain insurance and we could default under our debt our instruments if the cost and/or availability of certain types of insurance make it impractical or impossible to comply with such covenants relating to insurance. Belmont providesand Ticonderoga provide coverage solely on properties owned by the Company or its affiliates.

Furthermore, with respect to certain of our properties, including properties held by joint ventures or subject to triple net leases, insurance coverage is obtained by a third-party and we do not control the coverage. While we may have agreements with such third parties to maintain adequate coverage and we monitor these policies, such coverage ultimately may not be maintained or adequately cover our risk of loss.

Our operations are primarily concentrated in the New York metropolitan area. In the aftermath of a terrorist attack or other acts of terrorism or war, tenants in the New York metropolitan area may choose to relocate their business to less populated, lower-profile areas of the United States that those tenants believe are not as likely to be targets of future terrorist activity. In addition, economic activity could decline as a result of terrorist attacks or other acts of terrorism or war, or the perceived threat of such acts. Each of these impacts could in turn trigger a decrease in the demand for space in the New York metropolitan area, which could increase vacancies in our properties and force us to lease our properties on less favorable terms. While under the Terrorism Risk Insurance Program Reauthorization Act of 2015, insurers must make terrorism insurance available under their property and casualty insurance policies, this legislation does not regulate the pricing of such insurance. The absence of affordable terrorism insurance coverage may adversely affect the general real estate lending market, lending volume and the market's overall liquidity and, in the event of an uninsured loss, we could lose all or a portion of our assets. Furthermore, we may also experience increased costs in relation to security equipment and personnel. As a result, the value of our properties and our results of operations could materially decline.

We are subject to risks associated with natural disasters and the physical effects of climate change, which can include storms, hurricanes and flooding, any of which could have a material adverse effect on our properties, operations and business. To the extent climate change causes changes in weather patterns, our markets could experience increases in storm intensity and rising sea-levels. Over time, these conditions could result in declining demand for office space in our buildings or the inability of us to operate the buildings at all. Climate change may also have indirect effects on our business by increasing the cost of (or making unavailable) property insurance on terms we find acceptable, increasing the cost of energy at our properties and requiring us to expend funds as we seek to repair and protect our properties against such risks. There can be no assurance that climate change will not have a material adverse effect on our properties, operations or business.

Substantially all of our assets are held through subsidiaries of our Operating Partnership. Our Operating Partnership’s cash flow is dependent on cash distributions to it byfrom its subsidiaries, and in turn, substantially all of our cash flow is dependent on cash distributions to us byfrom our Operating Partnership. The creditors of each of our direct and indirect subsidiaries are entitled to payment of that subsidiary’s obligations to them, when due and payable, before distributions may be made by that subsidiary to its equity holders.

Therefore, our Operating Partnership’s ability to make distributions to holders of its partnership units depends on its subsidiaries’ ability first to satisfy their obligations to their creditors and then to make distributions to our Operating Partnership. Likewise, our ability to pay dividends to holders of common stock and preferred stock depends on our Operating Partnership’s ability first to satisfy its obligations to its creditors and make distributions payable to holders of preferred units and then to make distributions to us.

Furthermore, the holders of preferred partnership units of our Operating Partnership are entitled to receive preferred distributions before payment of distributions to holders of common units of our Operating Partnership, including us. Thus, our ability to pay cash dividends to our shareholders and satisfy our debt obligations depends on our Operating Partnership’s ability

In addition, our participation in any distribution of the assets of any of our direct or indirect subsidiaries upon any liquidation, reorganization or insolvency is only after the claims of the creditors, including trade creditors and preferred security holders, are satisfied.

Cash flow could be insufficient to pay dividends and meet the payments of principal and interest required under our current mortgages, our 2017 credit facility, our senior unsecured notes, our debentures and indebtedness outstanding at our joint venture properties. The total principal amount of our outstanding consolidated indebtedness was $5.9$5.6 billion as of December 31, 2017,2019, consisting of $1.5 billion in unsecured bank term loans (or "Term Loan A" and "Term Loan B"), $1.4$1.5 billion under our senior unsecured notes, $0.1 billion of junior subordinated deferrable interest debentures, $2.9$2.2 billion of non-recourse mortgages and loans payable on certain of our properties and debt and preferred equity investments, $40.0$240.0 million drawn under our revolving credit facility, and $11.8 million of outstanding letters of credit. In addition, we could increase the amount of our outstanding consolidated indebtedness in the future, in part by borrowing under the revolving credit facility portion of our 2017 credit facility. As of December 31, 2017,2019, the total principal amount of non-recourse indebtedness outstanding at the joint venture properties was $9.5$9.0 billion, of which our proportionate share was $4.2$4.0 billion. As of December 31, 2017,2019, we had no recourse indebtedness outstanding at our unconsolidated joint venture properties.

If we are unable to make payments under our 2017 credit facility, all amounts due and owing at such time shall accrue interest at a rate equal to 2% higher than the rate at which each draw was made. If we are unable to make payments under our senior unsecured notes, the principal and unpaid interest will become immediately payable. If a property is mortgaged to secure payment of indebtedness and we are unable to meet mortgage payments, the mortgagee could foreclose on the property, resulting in loss of income and asset value. Foreclosure on mortgaged properties or an inability to make payments under our 2017 credit facility or our senior unsecured notes could trigger defaults under the terms of our other financings, making such financings at risk of being declared immediately payable, and would have a negative impact on our financial condition and results of operations.

We may not be able to refinance existing indebtedness, which may require substantial principal payments at maturity. $164.8$55.0 million of consolidated mortgage debt and $187.7$105.0 million of unconsolidated joint venture debt is scheduled to mature in 20182020 after giving effect to our as-of-right extension options and repayments and refinancing of consolidated and joint venture debt between December 31, 20172019 and February 22, 201827, 2020 as discussed in the "Financial Statements and Supplementary Data" section. At the present time, we intend to repay, refinance, or exercise extension options on the debt associated with our properties on or prior to their respective maturity dates. At the time of refinancing, prevailing interest rates or other factors, such as the possible reluctance of lenders to make commercial real estate loans, may result in higher interest rates. Increased interest expense on the extended or refinanced debt would adversely affect cash flow and our ability to service debt obligations and pay dividends and distributions to security holders. If any principal payments due at maturity cannot be repaid, refinanced or extended, our cash flow will not be sufficient to repay maturing or accelerated debt.

The mortgages and mezzanine loans on our properties generally contain customary negative covenants that limit our ability to further mortgage the properties, to enter into material leases without lender consent or materially modify existing leases, among other things. In addition, our 2017 credit facility and senior unsecured notes contain restrictions and requirements on our method of operations. Our 2017 credit facility and our unsecured notes also require us to maintain designated ratios, including but not limited to, total debt-to-assets, debt service coverage and unencumbered assets-to-unsecured debt. These restrictions could adversely affect operations (including reducing our flexibility and our ability to incur additional debt), our ability to pay debt obligations and our ability to pay dividends and distributions to security holders.

Advances under our 2017 credit facility and certain property-level mortgage debt bear interest at a variable rate. Our consolidated variable rate borrowings totaled $1.6$2.0 billion at December 31, 2017.2019. In addition, we could increase the amount of our outstanding variable rate debt in the future, in part by borrowing additional amounts under our 2017 credit facility. Borrowings under our revolving credit facility and term loans bore interest at the 30-day LIBOR, plus spreads of 100 basis points, 110 basis points, and 165100 basis points, respectively, at December 31, 2017.2019. As of December 31, 2017,2019, borrowings under our term loans and junior subordinated deferrable interest debentures totaled $1.5 billion and $100.0 million, respectively. We may incur indebtedness in the future that also bears interest at a variable rate or may be required to refinance our debt at higher rates. At December 31, 2017,2019, a hypothetical 100 basis point increase in interest rates across each of our variable interest rate instruments, including our variable rate debt and preferred equity investments which mitigate our exposure to interest rate changes, would increase our net

The interest rate hedge instruments we use to manage some of our exposure to interest rate volatility involve risk and counterparties may fail to honor their obligationsperform under these arrangements. In addition, these arrangements may not be effective in reducing our exposure to interest rate changes and when existing interest rate hedges terminate, we may incur increased costs in putting in place further interest rate hedges. Failure to hedge effectively against interest rate changes may adversely affect our results of operations.

Our organizational documents do not contain any limitation on the amount of indebtedness we may incur. As of December 31, 2017, assuming the conversion of all outstanding units of the Operating Partnership into shares of our common stock, our combined debt-to-market capitalization ratio, including our share of joint venture debt, was 49.4%. Our market capitalization is variable and does not necessarily reflect the fair market value of our assets at all times. We also consider many factors other than market capitalization inwhen making decisions regarding the incurrence of indebtedness, such as the purchase price of properties to be acquired with debt financing, the estimated market value of our properties upon refinancing and the ability of particular properties and our business as a whole to generate cash flow to cover expected debt service. Any changes that increase our debt-to-market capitalization percentageleverage could be viewed negatively by investors. As a result, our stock price could decrease.

Our credit rating and the credit ratings assigned to our debt securities and our preferred stock could change based upon, among other things, our results of operations and financial condition. These ratings are subject to ongoing evaluation by credit rating agencies, and any rating could be changed or withdrawn by a rating agency in the future if, in its judgment, circumstances warrant such action. Moreover, these credit ratings are not recommendations to buy, sell or hold our common stock or any other securities. If any of the credit rating agencies that have rated our securities downgrades or lowers its credit rating, or if any credit rating agency indicates that it has placed any such rating on a “watch list” for a possible downgrading or lowering, or otherwise indicates that its outlook for that rating is negative, such action could have a material adverse effect on our costs and availability of funding, which could in turn have a material adverse effect on our financial condition, results of operations, cash flows, the trading price of our securities and our ability to satisfy our debt service obligations and to pay dividends and distributions to security holders.

We held first mortgages, mezzanine loans, junior participations and preferred equity interests with an aggregate net book value of $2.3$1.7 billion at December 31, 2017.2019. Some of these instruments may behave some recourse to their sponsors, while others are limited to the collateral securing the loan. In the event of a default under these obligations, we may have to take possession of the collateral securing these interests. Borrowers may contest enforcement of foreclosure or other remedies, seek bankruptcy protection against such enforcement and/or bring claims for lender liability in response to actions to enforce their obligations to us. Declines in the value of the property may prevent us from realizing an amount equal to our investment upon foreclosure or realization even if we make substantial improvements or repairs to the underlying real estate in order to maximize such property's investment potential. In addition, we may invest in mortgage-backed securities and other marketable securities.

We maintain and regularly evaluate the need for reserves to protect against potential future losses. Our reserves reflect management's judgment of the probability and severity of losses and the value of the underlying collateral. We cannot be certain that our judgment will prove to be correct and that our reserves will be adequate over time to protect against future losses because of unanticipated adverse changes in the economy or events adversely affecting specific properties, assets, tenants, borrowers, industries in which our tenants and borrowers operate or markets in which our tenants and borrowers or their properties are located. As

We co-invest with third parties through partnerships, joint ventures, co-tenancies or other structures, and by acquiring non-controlling interests in, or sharing responsibility for managing the affairs of, a property, partnership, joint venture, co-tenancy or other entity. Therefore, we may not be in a position to exercise sole decision-making authority regarding such property, partnership, joint venture or other entity. Investments in partnerships, joint ventures, or other entities may involve risks not present were a third

Each of our joint venture agreements has been individually negotiated with our partner in the joint venture and, in some cases, we have agreed to terms that are more favorable to our partner in the joint venture than to us. For example, our partner may be entitled to a specified portion of the profits of the joint venture before we are entitled to any portion of such profits. We may also enter into similar arrangements in the future. These rights may permit our partner in a particular joint venture to obtain a greater benefit from the value or profits of the joint venture than us, which could have an adverse effect on the value of our investment in the joint venture and on our financial condition and results of operations.

We are subject to various federal, state and local environmental and health and safety laws that can impose liability on current and former property owners or operators for the clean-up of certain hazardous substances released on a property or of contamination at any facility (e.g., a landfill) to which we have sent hazardous substances for treatment or disposal, without regard to fault or whether the release or disposal was in compliance with law. Being held responsible for such a clean-up could result in significant cost to us and have a material adverse effect on our financial condition and results of operations.

Our properties may be subject to risks relating to current or future laws, including laws benefiting disabled persons, such as the Americans with Disabilities Act, or ADA, and state or local zoning, construction or other regulations. Compliance with such laws may require significant property modifications in the future, which could be costly and non-compliance could result in fines being levied against us in the future. Such costs could have an adverse impact on our cash flows and ability to pay dividends to stockholders.

A change of control of our company could benefit stockholders by providing them with a premium over the then-prevailing market price of our stock. However, provisions contained in our charter and bylaws may delay or prevent a change in control of our company. These provisions, discussed more fully below, are:

To remain qualified as a REIT for federal income tax purposes, not more than 50% in value of our outstanding capital stock may be owned by five or fewer individuals at any time during the last half of any taxable year. For this purpose, stock may be "owned" directly, as well as indirectly under certain constructive ownership rules, including, for example, rules that attribute stock held by one shareholder to another shareholder. In part to avoid violating this rule regarding stock ownership limitations and maintain our REIT qualification, our charter prohibits ownership by any single stockholder of more than 9.0% in value or number of shares of our common stock. Limitations on the ownership of preferred stock may also be imposed by us.

Absent any exemption or waiver, stock acquired or held in excess of the limit on ownership will be transferred to a trust for the exclusive benefit of a designated charitable beneficiary, and the stockholder's rights to distributions and to vote would terminate. The stockholder would be entitled to receive, from the proceeds of any subsequent sale of the shares transferred to the charitable trust, the lesser of: the price paid for the stock or, if the owner did not pay for the stock, the market price of the stock on the date of the event causing the stock to be transferred to the charitable trust; and the amount realized from the sale.

This limitation on ownership of stock could delay or prevent a change in control of our company.

Under the Maryland General Corporation Law, or the MGCL, "business combinations" between a Maryland corporation and an interested stockholder or an affiliate of an interested stockholder are prohibited for five years after the most recent date on which the interested stockholder becomes an interested stockholder. These business combinations include a merger, consolidation, stock exchange or, in circumstances specified in the statute, an asset transfer or issuance or reclassification of equity securities. An interested stockholder is defined as:

A person is not an interested stockholder under the statute if the board of directors approves in advance the transaction by which he otherwise would have become an interested stockholder.

After the five-yearfive year prohibition, any business combination between the Maryland corporation and an interested stockholder generally must be recommended by the board of directors of the corporation and approved by the affirmative vote of at least:

The business combination statute may discourage others from trying to acquire control of us and increase the difficulty of consummating any offer, including potential acquisitions that might involve a premium price for our common stock or otherwise be in the best interest of our stockholders.

In addition, Maryland law provides that holders of "control shares" of a Maryland corporation acquired in a "control share acquisition" will not have voting rights with respect to the control shares except to the extent approved by a vote of two-thirds of the votes entitled to be cast on the matter, excluding shares of stock owned by the acquiror, by officers of the corporation or by directors who are employees of the corporation, under the Maryland Control Share Acquisition Act. "Control shares" means voting shares of stock that, if aggregated with all other shares of stock owned by the acquiror or in respect of which the acquiror is able to exercise or direct the exercise of voting power (except solely by virtue of a revocable proxy), would entitle the acquiror to exercise voting power in electing directors within one of the following ranges of voting power: (i) one-tenth or more but less than one-third; (ii) one-third or more but less than a majority; or (iii) a majority or more of all voting power. A "control share acquisition" means the acquisition of ownership of, or the power to direct the exercise of voting power with respect to, issued and outstanding control shares, subject to certain exceptions.

We have opted out of these provisions of the MGCL, with respect to business combinations and control share acquisitions, by resolution of our board of directors and a provision in our bylaws, respectively. However, in the future, our board of directors may reverse its decision by resolution and elect to opt in to the MGCL's business combination provisions, or amend our bylaws and elect to opt in to the MGCL's control share provisions.

Additionally, the MGCL permits our board of directors, without stockholder approval and regardless of what is provided in our charter or bylaws, to implement takeover defenses, some of which have not been implemented by our board of directors. Such takeover defenses, if implemented, may have the effect of inhibiting a third party from making us an acquisition proposal or of delaying, deferring or preventing a change in our control under circumstances that otherwise could provide our stockholders with an opportunity to realize a premium over the then-current market price.

Certain of our consolidated debt is not assumable and may be subject to significant prepayment penalties. These limitations could deter a change in control of our company.

Changing or new laws, regulations and standards relating to corporate governance and public disclosure, including SEC regulations and NYSE rules, can create uncertainty for public companies. These changed or new laws, regulations and standards are subject to varying interpretations in many cases due to their lack of specificity. As a result, their application in practice may evolve over time as new guidance is provided by regulatory and governing bodies, which could result in continuing uncertainty regarding compliance matters and higher costs necessitated by ongoing revisions to disclosure and governance practices. We are committed to maintaining high standards of corporate governance and public disclosure. If our efforts to comply with new or changed laws, regulations and standards differ from the activities intended by regulatory or governing bodies due to ambiguities related to practice, our reputation may be harmed.

Our efforts to comply with evolving laws, regulations and standards have resulted in, and are likely to continue to result in, increased general and administrative expenses and a diversion of management time and attention from revenue-generating activities to compliance activities. In particular, our continued efforts to comply with Section 404 of the Sarbanes-Oxley Act of 2002 and the related regulations regarding our required assessment of our internal controls over financial reporting and our external auditors' audit of that assessment have required the commitment of significant financial and managerial resources. We expect these efforts to require the continued commitment of significant resources. Further, our directors, chief executive officer and chief financial officer could face an increased risk of personal liability in connection with the performance of their duties. As a result, we may have difficulty attracting and retaining qualified directors and executive officers, which could harm our business.

Our charter authorizes itsour board of directors to issue additional shares of common stock, preferred stock and convertible equity or debt without stockholder approval and without the requirement to offer rights of pre-emption to existing stockholders. Any such issuance could dilute our existing stockholders' interests. Also, any future series of preferred stock may have voting provisions that could delay or prevent a change of control of our company.

As with other publicly traded equity securities, the value of our common stock depends on various market conditions, which may change from time to time. In addition to the current economic environment and future volatility in the securities and credit markets, the following market conditions may affect the value of our common stock:

The market value of our common stock is based on a number of factors including, but not limited to, the market's perception of the current and future value of our assets, our growth potential and our current and potential future earnings and cash dividends. Consequently, our common stock may trade at prices that are higher or lower than our net asset value per share of common stock.

Between January 1, 20172019 and December 31, 2017,2019, the closing sale price of our common stock on the New York Stock Exchange, or the NYSE, ranged from $94.15$76.79 to $113.75$93.47 per share. On February 22, 2018, the closing sale price of our common stock on the NYSE was $95.91. Our stock price may fluctuate in response to a number of events and factors, such as those described elsewhere in this "Risk Factors" section. Additionally, the amount of our leverage may hinder the demand for our common stock, which could have a material adverse effect on the market price of our common stock. Equity issuances or buybacks by us or the perception that such issuances or buybacks may occur may also affect the market price of our common stock.

There is a potential conflict of interest relating to the disposition of certain property contributed to us by Stephen L. Green and affiliated entities in our initial public offering. Mr. Green serves as a member and as the chairman emeritus of our board of directors and is an executive officer.directors. If we sell a property in a transaction in which a taxable gain is recognized, for tax purposes the built-in gain would be allocated solely to him and not to us. As a result, Mr. Green has a conflict of interest if the sale of a property he contributed is in our best interest but not his.

In addition, Mr. Green's tax basis includes his share of debt, including mortgage indebtedness, owed by the Operating Partnership. If the Operating Partnership were to retire such debt, then he would experience a decrease in his share of liabilities, which, for tax purposes, would be treated as a distribution of cash to him. To the extent the deemed distribution of cash exceeded his tax basis, he would recognize gain. As a result, Mr. Green has a conflict of interest if the refinancing of indebtedness is in our best interest but not his.

Alliance Building Services, or Alliance, and its affiliates are partially owned by Gary Green, a son of Stephen L. Green, who serves as a member and as the chairman emeritus of our board of directors, and provide services to certain properties owned by us. Alliance’s affiliates include First Quality Maintenance, L.P., or First Quality, Classic Security LLC, Bright Star Couriers LLC and Onyx Restoration Works, and provide cleaning, extermination, security, messenger, and restoration services, respectively. In addition, First Quality has the non-exclusive opportunity to provide cleaning and related services to individual tenants at our properties on a basis separately negotiated with any tenant seeking such additional services. The Service Corporation has entered into an arrangement with Alliance whereby it will receive a profit participation above a certain threshold for services provided by Alliance to certain tenants at certain buildings above the base services specified in their lease agreements.

Our company and our tenants accounted for 24.72%22.70% of Alliance's 20172019 estimated total revenue, based on information provided to us by Alliance. While we believe that the contracts pursuant to which these services are provided were the result of arm's length negotiations, there can be no assurance that the terms of such agreements, or dealings between the parties during the performance of such agreements, will be as favorable to us as those which could be obtained from unaffiliated third parties providing comparable services under similar circumstances.

The recently enacted Tax Cuts and Jobs Act (the ‘‘Tax Act’’) makes, signed into law on December 22, 2017, made substantial changes to the Code.  Among those changes are a significant permanent reduction in the generally applicable corporate tax rate, changes in the taxation of individuals and other non-corporate taxpayers that generally but not universally reduce their taxes on a temporary basis subject to ‘‘sunset’’ provisions, the elimination or modification of various currently allowed deductions (including substantial limitations on the deductibility of interest and, in the case of individuals, the deduction for personal state and local taxes), and preferential rates of taxation on most ordinary REIT dividends and certain business income derived by non-corporate taxpayers in comparison to other ordinary income recognized by such taxpayers. The Tax Act also imposes certain additional limitationsdid not have a material impact on our financial statements for the deduction of net operating losses, whichyears ended December 31, 2019 or December 31, 2018 and we do not expect that it will have significant direct impact on us.  However, it is possible that it could impact us indirectly, for example, because the overall investment thesis for REITs may be impacted in the future cause uspositive or negative ways that are difficult to be requiredpredict.  In addition, while some Treasury Regulations have been issued with respect to make distributions that will be taxable to our stockholders to the extent of our current or accumulated earnings and profits in order to comply with the annual REIT distribution requirements. The effect of these, and the many other, changes made in the Tax Act, is highly uncertain, both in terms of their direct effect on the taxation of an investment in our common stock and their indirect effect on the value of our assets or market conditions generally. Furthermore, many of the provisions of the Tax Act will require guidance through the issuance of Treasury regulations in order to assess their effect. Therethere may be a substantial delay before suchadditional regulations are promulgated, increasing the uncertainty as to the ultimate effect of the statutory amendments on us. It is also likely that there will be technical corrections legislation proposed with respect to the Act this year, the effect of which cannot be predicted and may be adverse to us or our stockholders.promulgated. 

Additionally, the rules dealing with U.S. federal income taxation are continually under review by Congress, the IRS, and the U.S. Department of the Treasury. Any such changes could have an adverse effect on an investment in our shares or on the market value or the resale potential of our assets.

We are dependent on the efforts of Marc Holliday, our chairman and chief executive officer, and Andrew W. Mathias, our president. These officers have employment agreements which expire in January 20192022 and December 2018,2021, respectively. A loss of the services of either of these individuals could adversely affect our operations and could be negatively perceived by the market resulting in a decrease in our stock price.

Despite system redundancy, the implementation of security measures and the existence of a disaster recovery plan for our internal information technology systems, our systems are vulnerable to a number of risks including energy blackouts, natural disasters, terrorism, war, telecommunication failures and cyber attacks and intrusions, such as computer viruses, malware, attachments to e-mails, intrusion and unauthorized access, including from persons inside our organization or from persons outside our organization with access to our systems. The risk of a security breach or disruption, particularly through cyber attacks and intrusions, including by computer hackers, foreign governments and cyber terrorists, has generally increased as the number, intensity and sophistication of attempted attacks and instructions from around the world have increased. Our systems are critical to the operation of our business and any system failure, accident or security breach that causes interruptions in our operations could result in a material disruption to our business. We may also incur additional costs to remedy damages caused by such disruptions. Although we make efforts to maintain the security and integrity of our systems and have implemented various measures to manage the risk of a security breach or disruption, there can be no assurance that our security efforts and measures will be effective or that attempted security breaches or disruptions would not be successful or damaging. Any compromise of our security could also result in a violation of applicable privacy and other laws, significant legal and financial exposure, damage to our reputation, loss or misuse of the information (which may be confidential, proprietary and/or commercially sensitive in nature) and a loss of confidence in our security measures, which could harm our business.

See Item 7, "Management's Discussion and Analysis of Financial Condition and Results of Operations—Forward-looking Information," for additional disclosure regarding forward-looking statements.

As of December 31, 2017,2019, we did not have any unresolved comments with the staff of the SEC.