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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, 2015
Dec. 31, 2014
Dec. 31, 2013
Total real estate investments:      
Balance, beginning of year $ 1,560,159 $ 1,459,439 $ 1,466,068
Acquisitions 4,894 70,921 5,124
Improvements 70,067 34,417 21,991
Retirements (1,981) (4,618) (33,744)
Transfers to assets held for sale (10,429) 0 0
Balance, end of year 1,622,710 1,560,159 1,459,439
Total accumulated depreciation:      
Balance, beginning of year 396,617 364,663 353,305
Depreciation expense 37,698 35,933 43,204
Retirements (1,911) (3,979) (31,846)
Transfers to assets held for sale (7,034) 0 0
Balance, end of year $ 425,370 $ 396,617 $ 364,663