We use the deferral method to account for drydocking costs. Under the deferral method, drydocking costs incurred are deferred and charged to expense on a straight-line basis over the period to the next scheduled drydock, which we estimate to be a period of thirty to sixty months based on the vessel's age as required by Class. Deferred drydock costs consist of the costs to drydock the vessel and other costs incurred in connection with the drydock which are necessary to maintain the vessel's Class certification. Class certification is necessary in order for our cruise ships to be flagged in a specific country, obtain liability insurance and legally operate as passenger cruise ships. The activities associated with those drydocking costs cannot be performed while the vessel is in service and, as such, are done during a drydock as a planned major maintenance activity. The significant deferred drydock costs consist of hauling and wharfage services provided by the drydock facility, hull inspection and related activities (e.g., scraping, pressure cleaning, bottom painting), maintenance to steering propulsion, thruster equipment and ballast tanks, port services such as tugs, pilotage and line handling, and freight associated with these items. We perform a detailed analysis of the various activities performed for each drydock and only defer those costs that are directly related to planned major maintenance activities necessary to maintain Class. The costs deferred are related to activities not otherwise routinely periodically performed to maintain a vessel's designed and intended operating capability. Repairs and maintenance activities are charged to expense as incurred.

We use judgment when estimating the period between drydocks, which can result in adjustments to the estimated amortization of drydock costs. If the vessel is disposed of before the next drydock, the remaining balance in deferred drydock is written-off to the gain or loss upon disposal of vessel in the period in which the sale takes place. We also use judgment when identifying costs incurred during a drydock which are necessary to maintain the vessel's Class certification as compared to those costs attributable to repairs and maintenance which are expensed as incurred.

We believe we have made reasonable estimates for ship accounting purposes. However, should certain factors or circumstances cause us to revise our estimates of ship useful lives or projected residual values, depreciation expense could be materially higher or lower. If circumstances cause us to change our assumptions in making determinations as to whether ship improvements should be capitalized, the amounts we expense each year as repairs and maintenance costs could increase, partially offset by a decrease in depreciation expense. If we had reduced our estimated average ship useful life by one year, depreciation expense for 20152018 would have increased by approximately $66.7$63.8 million. If our ships were estimated to have no residual value, depreciation expense for 20152018 would have increased by approximately $188.8$243.0 million.

We review goodwill trademarks and trade names, which are our most significant indefinite-lived intangible assets for impairment at the reporting unit level annually or, when events or circumstances dictate, more frequently. The impairment review for goodwill consists of a qualitative assessment of whether it is more-likely-than-not that a reporting unit's fair value is less than its carrying amount, and if necessary, a two-step goodwill impairment test. Factors to consider when performing the qualitative assessment include general economic conditions, limitations on accessing capital, changes in forecasted operating results, changes in fuel prices and fluctuations in foreign exchange rates. If the qualitative assessment demonstrates that it is more-likely-than-not that the estimated fair value of the reporting unit exceeds its carrying value, it is not necessary to perform the two-step goodwill impairment test. We may elect to

When performing the two-step goodwill impairment test, the fair value of the reporting unit is determined and compared to the carrying value of the net assets allocated to the reporting unit. We estimate the fair value of our reporting units using a probability-weighted discounted cash flow model. The estimation of fair value utilizing discounted expected future cash flows includes numerous uncertainties which require our significant judgment when making assumptions of expected revenues, operating costs, marketing, selling and administrative expenses, interest rates, ship additions and retirements as well as assumptions regarding the cruise vacation industry's competitive environment and general economic and business conditions, among other factors. The principal assumptions usedwe use in the discounted cash flow model are projected operating results, weighted-average cost of capital, and terminal value. The discounted cash flow model uses our 2016the most current projected operating results for the upcoming fiscal year as a base. To that base, we add future years' cash flows assuming multiple revenue and expense scenarios that reflect the impact of different global economic environments beyond 2016the base year on the reporting unit. We discount the projected cash flows using rates specific to the reporting unit based on its weighted-average cost of capital. If the fair value of the reporting unit exceeds its carrying value, no further analysis or write-down of goodwill is required. If the fair value of the reporting unit is less than the carrying value of its net assets, the implied fair value of the reporting unit is allocated to all its underlying assets and liabilities, including both recognized and unrecognized tangible and intangible assets, based on their fair value. If necessary, goodwill is then written down to its implied fair value.

We review our ships aircraft and other long-lived assets for impairment whenever events or changes in circumstances indicate, based on estimated undiscounted future cash flows, that the carrying amount of these assets may not be fully recoverable. We evaluate asset impairment at the lowest level for which identifiable cash flows are largely independent of the cash flows of other assets and liabilities. The lowest level for which we maintain identifiable cash flows that are independent of the cash flows of other assets and liabilities is at the ship level for our ships and, prior to the sale of the aircraft, at the aggregated asset group level for our aircraft. If estimated future cash flows are less than the carrying value of an asset, an impairment charge is recognized to the extent its carrying value exceeds fair value.

We estimate fair value based on quoted market prices in active markets, if available. If active markets are not available, we base fair value on independent appraisals, sales price negotiations and projected future cash flows discounted at a rate estimated by management to be commensurate with the business risk. Quoted market prices are often not available for individual reporting units and for indefinite-life intangible assets. Accordingly, we estimate the fair value of a reporting unit and an indefinite-life intangible asset using an expected present value technique.