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Real Estate and Accumulated Depreciation - Changes in Total Real Estate Investments and Related Accumulated Depreciation (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Total real estate investments:      
Balance, beginning of year $ 1,803,200 $ 1,700,813 $ 1,622,710
Acquisitions 48,579 77,258 48,123
Improvements 98,917 42,640 35,826
Retirements (2,531) (17,511) (5,846)
Balance, end of year 1,948,165 1,803,200 1,700,813
Total accumulated depreciation:      
Balance, beginning of year 488,166 458,279 425,370
Depreciation expense 39,768 40,197 38,755
Retirements (2,416) (10,310) (5,846)
Balance, end of year $ 525,518 $ 488,166 $ 458,279