Segment Information (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Segment Information
Rental income	$ 251,246	$ 210,673
Operating expenses	(104,090)	(90,397)
Property net operating income (NOI)	147,156	120,276
Reconciliation of NOI to net income
Property NOI	147,156	120,276
Depreciation and amortization	(61,351)	(52,289)
General and administrative	(12,310)	(10,959)
Acquisition related costs	(2,502)	(2,559)
Operating income	70,993	54,469
Interest and other income	288	708
Interest expense	(49,106)	(47,414)
Loss on early extinguishment of debt	(67)
Equity in earnings of investees	2,958	2,712
Income from continuing operations before income tax expense	25,066	10,475
Income tax expense	(492)	(346)
Income from continuing operations	24,574	10,129
Income from discontinued operations		1,911
Net gain on sale of properties from discontinued operations		34,572
Net income	$ 24,574	$ 46,612