Basis of Presentation (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Dec. 31, 2010
Assets [Abstract]
Cash and cash equivalents attributable to consolidated VIEs	$ 432,520	$ 224,964	$ 432,520	$ 224,964	$ 180,726	$ 162,745
Other Assets attributable to consolidated VIEs	92,178		92,178		86,176
Total assets	3,484,016		3,484,016		3,045,812
Liabilities [Abstract]
Distributions payable not attributable to consolidated VIE	38,534		38,534		32,288
Total liabilities	1,460,607		1,460,607		1,318,353
Gross Revenues	68,721	44,878	134,525	87,036
Net Investments In Real Estate	2,513,385		2,513,385		2,374,773
International [Member]
Liabilities [Abstract]
Gross Revenues	20,764	13,474	41,636	25,838
Net Investments In Real Estate	967,770	980,160	967,770	980,160
Parent [Member]
Assets [Abstract]
Cash and cash equivalents attributable to consolidated VIEs	707		707		7,585
Other Assets attributable to consolidated VIEs	1,256		1,256		2,811
Total assets	1,963		1,963		10,396
Liabilities [Abstract]
Due to affiliates not attributable to consolidated VIE	(2,023)		(2,023)		(1,044)
Distributions payable not attributable to consolidated VIE	(38,534)		(38,534)		(32,288)
Total liabilities	$ (40,557)		$ (40,557)		$ (33,332)