Real Estate and Accumulated Depreciation Activity of Real Estate and Accumulated Depreciation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Real Estate, Balance at the beginning of the year	$ 20,114,576	$ 19,451,683	$ 19,208,417
Real Estate, Additions to / improvements of real estate	1,099,286	977,287	700,792
Real Estate, Assets sold / written off	(155,148)	(314,394)	(457,526)
Real Estate, Balance at the end of the year	21,058,714	20,114,576	19,451,683
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Accumulated depreciation, Balance at beginning of the year	4,201,891	3,905,940	3,529,978
Accumulated depreciation, Depreciation expense	497,059	560,024	486,450
Accumulated depreciation, Assets sold / written off	(132,380)	(264,073)	(110,488)
Accumulated depreciation, Balance at end of the year	$ 4,566,570	$ 4,201,891	$ 3,905,940