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Real Estate and Accumulated Depreciation - Changes in Total Real Estate Investments and Related Accumulated Depreciation (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Total real estate investments:      
Balance, beginning of year $ 1,466,068 $ 1,417,845 $ 1,224,036
Acquisitions 5,124 34,247 168,905
Improvements 21,991 23,095 25,955
Retirements (33,744) (9,119) (1,051)
Balance, end of year 1,459,439 1,466,068 1,417,845
Total accumulated depreciation:      
Balance, beginning of year 353,305 326,397 296,786
Depreciation expense 43,204 34,565 30,555
Retirements (31,846) (7,657) (944)
Balance, end of year $ 364,663 $ 353,305 $ 326,397