Segment Information (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2013	Mar. 31, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Information
Rental income	$ 274,766		$ 249,797	$ 549,814	$ 493,175
Operating expenses	(109,754)		(103,034)	(219,413)	(200,270)
Property net operating income (NOI)	165,012		146,763	330,401	292,905
Reconciliation of NOI to net income
Property NOI	165,012		146,763	330,401	292,905
Depreciation and amortization	(67,389)		(60,433)	(133,912)	(118,452)
General and administrative	(21,653)		(12,595)	(38,919)	(24,131)
Acquisition related costs	(145)		(1,434)	(773)	(3,936)
Operating income	75,825		72,301	156,797	146,386
Interest and other income	250		360	708	645
Interest expense	(43,762)		(50,237)	(96,106)	(99,343)
Loss on early extinguishment of debt		(60,027)	(1,608)	(60,027)	(1,675)
Equity in earnings of investees	159		2,829	4,421	5,787
Gain on sale of equity investment				66,293
Income from continuing operations before income tax expense	32,472		23,645	72,086	51,800
Income tax expense	(754)		(92)	(1,742)	(584)
Income from continuing operations	31,718		23,553	70,344	51,216
Loss from discontinued operations	(311)		(3,317)	(2,223)	(6,406)
Loss on asset impairment from discontinued operations	(4,589)			(8,535)
Gain on sale of properties from discontinued operations	2,099		350	3,359	350
Income before gain on sale of properties	28,917		20,586	62,945	45,160
Gain on sale of properties				1,596
Net income	$ 28,917		$ 20,586	$ 64,541	$ 45,160