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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, 2016
Dec. 31, 2015
Dec. 31, 2014
Total real estate investments:      
Balance, beginning of year $ 1,622,710 $ 1,560,159 $ 1,459,439
Acquisitions 48,123 4,894 70,921
Improvements 35,826 70,067 34,417
Retirements (5,846) (1,981) (4,618)
Transfers to assets held for sale 0 (10,429) 0
Balance, end of year 1,700,813 1,622,710 1,560,159
Total accumulated depreciation:      
Balance, beginning of year 425,370 396,617 364,663
Depreciation expense 38,755 37,698 35,933
Retirements (5,846) (1,911) (3,979)
Transfers to assets held for sale 0 (7,034) 0
Balance, end of year $ 458,279 $ 425,370 $ 396,617