Real Estate and Accumulated Depreciation Activity of Real Estate and Accumulated Depreciation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Real Estate, Balance at the beginning of the year	$ 19,451,683	$ 19,208,417	$ 18,953,601
Real Estate, Additions to / improvements of real estate	977,287	700,792	594,296
Real Estate, Assets sold / written off	(314,394)	(457,526)	(339,480)
Real Estate, Balance at the end of the year	20,114,576	19,451,683	19,208,417
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Accumulated depreciation, Balance at beginning of the year	3,905,940	3,529,978	3,145,701
Accumulated depreciation, Depreciation expense	560,024	486,450	456,176
Accumulated depreciation, Assets sold / written off	(264,073)	(110,488)	(71,899)
Accumulated depreciation, Balance at end of the year	$ 4,201,891	$ 3,905,940	$ 3,529,978