Real Estate and Accumulated Depreciation Activity of Real Estate and Accumulated Depreciation (Details) - USD ($) $ in Thousands	12 Months Ended
	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