Segment Information (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment Information
Rental income	$ 261,661	$ 241,785	$ 768,281	$ 670,396
Operating expenses	(112,577)	(104,683)	(324,760)	(285,703)
Property net operating income (NOI)	149,084	137,102	443,521	384,693
Reconciliation of NOI to net income
Property NOI	149,084	137,102	443,521	384,693
Depreciation and amortization	(63,437)	(56,389)	(188,340)	(159,072)
General and administrative	(14,592)	(11,692)	(40,266)	(34,275)
Loss on asset impairment		(9,247)		(9,247)
Acquisition related costs	(1,066)	(4,805)	(5,002)	(9,722)
Operating income	69,989	54,969	209,913	172,377
Interest and other income	399	320	1,100	1,395
Interest expense	(51,138)	(49,423)	(150,481)	(145,037)
(Loss) gain on early extinguishment of debt	(220)	310	(1,895)	310
Equity in earnings of investees	2,868	2,768	8,655	8,390
Gain on issuance of shares by an equity investee		11,177		11,177
Income from continuing operations before income tax expense	21,898	20,121	67,292	48,612
Income tax expense	(1,322)	(307)	(1,906)	(743)
Income from continuing operations	20,576	19,814	65,386	47,869
Income from discontinued operations		1,720		5,669
Net gain on sale of properties from discontinued operations		7,001		41,573
Income before gain on sale of properties	20,576	28,535	65,386	95,111
Gain on sale of properties	1,689		2,039
Net income	$ 22,265	$ 28,535	$ 67,425	$ 95,111