We provide for depreciation and amortization using the straight-line method over the following estimated useful asset lives:



Classification	Estimated Useful Asset Life
Buildings	42 years
Leasehold improvements	Over the shorter of the asset’s life or the lease term
Furniture, fixtures, equipment and software	3 to 14 years
Assets held under finance leases	Over the shorter of the asset’s life or the lease term

Depreciation expense was $36.1 million, $37.3 million, and $38.8 million in 2019, 2018, and 2017, respectively.

At the time property is retired or otherwise disposed of, the asset and related accumulated depreciation are removed from the accounts, and any resulting gain or loss is credited or charged to other income, net.

Assets held under finance leases, consisting primarily of information technology equipment, are recorded in property with the corresponding obligations carried in long-term debt. The amount capitalized is the present value at the beginning of the lease term of the aggregate future minimum lease payments. Assets capitalized as finance leases during the year ended December 31, 2019 and 2018 were $2.0 million and $3.1 million, respectively.

We capitalize interest expense on major construction and development projects while in progress. Interest capitalized for 2019, 2018, and 2017 was $0.2 million, $0.3 million and $0.1 million, respectively.

Where applicable, we capitalize qualifying internal and external costs incurred to develop or obtain software for internal use during the application development stage. Costs incurred during the pre-application development and post-implementation stages are expensed as incurred. We capitalized software and software development costs of $3.0 million and $3.7 million in 2019 and 2018, respectively, and the amounts are recorded in software.

We review long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of the asset may not be recoverable. For assets classified as held and used, impairment may occur if projected undiscounted cash flows are not adequate to cover the carrying value of the assets. In such cases, additional analysis is conducted to determine the amount of the loss to be recognized. The impairment loss is calculated as the difference between the carrying amount of the asset and its estimated fair value. The analysis requires estimates of the amount and timing of projected cash flows and, where applicable, selection of an appropriate discount rate. Such estimates are critical in determining whether any impairment charge should be recorded and the amount of such charge if an impairment loss is deemed necessary.

There were no impairment losses related to property recorded during 2019 or 2017. During 2018, we recorded an impairment loss of $0.3 million to account for the expected loss on an abandoned property that did not qualify as an asset held for sale, where the net book value of the property exceeded the estimated selling price less estimated selling expenses. The impairment loss is included in other income, net in the consolidated statements of income.