Segment Information (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment Information
Rental income	$ 255,374	$ 217,938	$ 506,620	$ 428,611
Operating expenses	(108,093)	(90,623)	(212,183)	(181,020)
Property net operating income (NOI)	147,281	127,315	294,437	247,591
Reconciliation of NOI to net income
Property NOI	147,281	127,315	294,437	247,591
Depreciation and amortization	(63,552)	(50,394)	(124,903)	(102,683)
General and administrative	(13,364)	(11,624)	(25,674)	(22,583)
Acquisition related costs	(1,434)	(2,358)	(3,936)	(4,917)
Operating income	68,931	62,939	139,924	117,408
Interest and other income	413	367	701	1,075
Interest expense	(50,237)	(48,200)	(99,343)	(95,614)
Loss on early extinguishment of debt	(1,608)		(1,675)
Equity in earnings of investees	2,829	2,910	5,787	5,622
Income from continuing operations before income tax expense	20,328	18,016	45,394	28,491
Income tax expense	(92)	(90)	(584)	(436)
Income from continuing operations	20,236	17,926	44,810	28,055
Income from discontinued operations		2,038		3,949
Net gain on sale of properties from discontinued operations				34,572
Income before gain on sale of properties	20,236	19,964	44,810	66,576
Gain (loss) on sale of properties	350		350
Net income	$ 20,586	$ 19,964	$ 45,160	$ 66,576