Schedule III Real Estate and Accumulated Depreciation (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Activity in real estate
Balance at end of year	$ 2,573,731	$ 2,198,361
Activity in accumulated depreciation
Balance at end of year	319,302	263,042
Net real estate assets	2,263,795	1,972,402	2,049,859
Aggregate cost of real estate for U.S. federal income tax purposes	2,117,151
Operating facilities
Activity in real estate
Balance at beginning of year	2,198,361	2,249,262	2,121,257
Acquisitions	301,531	89,750	21,764
Improvements	39,352	16,563	31,652
Transfers	34,777	33,407	78,148
Dispositions and other	(290)	(190,621)	(3,559)
Balance at end of year	2,573,731	2,198,361	2,249,262
Activity in accumulated depreciation
Balance at beginning of year	263,042	233,830	182,335
Depreciation expense	56,702	48,665	50,530
Dispositions and other	(442)	(19,453)	965
Balance at end of year	319,302	263,042	233,830
Construction in progress
Activity in real estate
Balance at beginning of year	37,083	34,427	58,734
Current development	7,060	36,063	67,301
Transfers	(34,777)	(33,407)	(78,148)
Dispositions and other			(13,460)
Balance at end of year	$ 9,366	$ 37,083	$ 34,427