Consolidated Statements of Income (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Rental income	$ 911,948	$ 782,463	$ 764,461
Expenses:
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
Loss on asset impairment	10,355	127,740	15,179
Acquisition related costs	10,073	21,553	4,082
Total expenses	678,005	729,284	557,948
Operating income	233,943	53,179	206,513
Interest and other income	1,718	2,999	1,195
Interest expense (including net amortization of debt discounts, premiums and deferred financing fees of $6,943, $7,150 and $6,124, respectively)	(195,024)	(179,642)	(166,855)
(Loss) gain on early extinguishment of debt	(35)	(796)	20,686
Equity in earnings of investees	11,377	8,464	6,546
Gain on issuance of shares by an equity investee	11,177	34,808
Gain on asset acquisition		20,392
Income (loss) from continuing operations before income tax expense	63,156	(60,596)	68,085
Income tax expense	(1,347)	(550)	(735)
Income (loss) from continuing operations	61,809	(61,146)	67,350
Discontinued operations:
Income from discontinued operations	5,423	26,223	34,894
Loss on asset impairment from discontinued operations		(1,524)	(16,703)
Loss on early extinguishment of debt from discontinued operations		(248)
Net gain on sale of properties from discontinued operations	42,752	137,768	79,133
Income before gain on sale of properties	109,984	101,073	164,674
Gain on sale of properties		34,336
Net income	109,984	135,409	164,674
Preferred distributions	(46,985)	(47,733)	(50,668)
Excess redemption price paid over carrying value of preferred shares		(5,921)
Net income available for common shareholders	$ 62,999	$ 81,755	$ 114,006
Weighted average common shares outstanding - basic (in shares)	77,428	64,703	56,055
Weighted average common shares outstanding - diluted (in shares)	84,726	72,001	63,353
Basic and diluted earnings per common share:
Income (loss) from continuing operations available for common shareholders (in dollars per share)	$ 0.19	$ (1.24)	$ 0.30
Income from discontinued operations (in dollars per share)	$ 0.62	$ 2.51	$ 1.74
Net income available for common shareholders (in dollars per share)	$ 0.81	$ 1.26	$ 2.03