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Real Estate and Accumulated Depreciation Activity of Real Estate and Accumulated Depreciation (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward] |  |  |
Real Estate, Balance at the beginning of the year | $ 19,208,417 | $ 18,953,601 | $ 14,869,887
Real Estate, Additions to / improvements of real estate | 700,792 | 594,296 | 4,410,622
Real Estate, Assets sold / written off | (457,526) | (339,480) | (326,908)
Real Estate, Balance at the end of the year | 19,451,683 | 19,208,417 | 18,953,601
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward] |  |  |
Accumulated depreciation, Balance at beginning of the year | 3,529,978 | 3,145,701 | 2,919,479
Accumulated depreciation, Depreciation expense | 486,450 | 456,176 | 419,908
Accumulated depreciation, Assets sold / written off | (110,488) | (71,899) | (193,686)
Accumulated depreciation, Balance at end of the year | $ 3,905,940 | $ 3,529,978 | $ 3,145,701

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