Real Estate and Accumulated Depreciation Activity of Real Estate and Accumulated Depreciation (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Real Estate, Balance at the beginning of the year	$ 13,363,113	$ 12,740,892	$ 11,075,879
Real Estate, Additions to / improvements of real estate	1,602,583	668,084	1,669,926
Real Estate, Assets sold / written off	(95,809)	(45,863)	(4,913)
Real Estate, Balance at the end of the year	14,869,887	13,363,113	12,740,892
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Accumulated depreciation, Balance at beginning of the year	2,626,324	2,308,665	2,020,056
Accumulated depreciation, Depreciation expense	367,625	362,636	292,561
Accumulated depreciatio, Assets sold / written off	(74,470)	(44,977)	(3,952)
Accumulated depreciation, Balance at end of the year	$ 2,919,479	$ 2,626,324	$ 2,308,665