Property and equipment, net consisted of the following (in thousands):

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The increase in ships was primarily due to the addition of Norwegian Encore. The Company capitalized approximately $458.9 million of costs associated with ship improvements. Depreciation expense for the years ended December 31, 2019, 2018 and 2017 was $627.7 million, $534.9 million and $478.7 million, respectively. Repairs and maintenance expenses including Dry-dock expenses were $199.7 million, $199.5 million and $157.2 million for the years ended December 31, 2019, 2018 and 2017, respectively, and were recorded within other cruise operating expense.

Ships under construction include progress payments to the shipyard, planning and design fees and other associated costs. Capitalized interest costs which were primarily associated with the construction or revitalization of ships amounted to $32.9 million, $30.4 million and $29.0 million for the years ended December 31, 2019, 2018 and 2017, respectively.