Real Estate Properties (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Income Statements:
Rental income	$ 241,552	$ 241,785	$ 217,938	$ 210,673	$ 200,645	$ 196,272	$ 192,930	$ 192,616	$ 911,948	$ 782,463	$ 764,461
Operating expenses									(392,131)	(333,049)	(324,050)
Depreciation and amortization									(218,688)	(207,205)	(178,034)
General and administrative									(46,758)	(39,737)	(36,603)
Operating income									233,943	53,179	206,513
Interest income									1,718	2,999	1,195
Interest expense									(195,024)	(179,642)	(166,855)
Income from discontinued operations									5,423	26,223	34,894
Discontinued operations, properties sold or held for sale
Income Statements:
Rental income									20,460	80,973	97,222
Operating expenses									(9,726)	(31,802)	(34,596)
Depreciation and amortization									(4,463)	(16,323)	(17,647)
General and administrative									(722)	(2,834)	(3,268)
Acquisition related costs									(128)	(7)	(216)
Operating income									5,421	30,007	41,495
Interest income									2	7	2
Interest expense										(3,791)	(6,603)
Income from discontinued operations									$ 5,423	$ 26,223	$ 34,894