Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2010		Sep. 30, 2010		Jun. 30, 2010		Mar. 31, 2010		Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Quarterly Financial Information Disclosure [Abstract]
Real estate rental revenue	$ 76,708	[1],[2]	$ 71,931	[1],[2]	$ 71,684	[1],[2]	$ 69,204	[1],[2]	$ 65,364	[1],[2]	$ 65,032	[1],[2]	$ 64,087	[1],[2]	$ 64,007	[1],[2]	$ 289,527	$ 258,490	$ 256,549
Income (loss) from continuing operations	(10,385)	[1],[2]	2,707	[1],[2]	4,431	[1],[2]	2,119	[1],[2]	(6,963)	[1],[2]	3,093	[1],[2]	3,444	[1],[2]	1,418	[1],[2]	(1,128)	992	9,723
Net income	31,098	[1],[2]	63,036	[1],[2]	6,556	[1],[2]	4,688	[1],[2]	10,615	[1],[2]	6,658	[1],[2]	15,021	[1],[2]	5,265	[1],[2]	105,378	37,559	40,948
Net income attributable to the controlling interests	$ 30,689	[1],[2]	$ 63,008	[1],[2]	$ 6,522	[1],[2]	$ 4,665	[1],[2]	$ 10,591	[1],[2]	$ 6,625	[1],[2]	$ 14,994	[1],[2]	$ 5,216	[1],[2]	$ 104,884	$ 37,426	$ 40,745
Earnings per common share, basic, Continuing operations	$ (0.16)	[1],[2]	$ 0.04	[1],[2]	$ 0.07	[1],[2]	$ 0.03	[1],[2]	$ (0.11)	[1],[2]	$ 0.05	[1],[2]	$ 0.05	[1],[2]	$ 0.02	[1],[2]	$ (0.02)	$ 0.02	$ 0.17
Earnings per common share, diluted, Continuing operations	$ (0.16)	[1],[2]	$ 0.04	[1],[2]	$ 0.07	[1],[2]	$ 0.03	[1],[2]	$ (0.11)	[1],[2]	$ 0.04	[1],[2]	$ 0.05	[1],[2]	$ 0.02	[1],[2]	$ (0.02)	$ 0.02	$ 0.17
Net income per share, Basic	$ 0.46	[1],[2]	$ 0.95	[1],[2]	$ 0.10	[1],[2]	$ 0.07	[1],[2]	$ 0.16	[1],[2]	$ 0.11	[1],[2]	$ 0.24	[1],[2]	$ 0.09	[1],[2]	$ 1.58	$ 0.60	$ 0.71
Net income per share, Diluted	$ 0.46	[1],[2]	$ 0.95	[1],[2]	$ 0.10	[1],[2]	$ 0.07	[1],[2]	$ 0.16	[1],[2]	$ 0.10	[1],[2]	$ 0.24	[1],[2]	$ 0.09	[1],[2]	$ 1.58	$ 0.60	$ 0.71

[1] With regard to per share calculations, the sum of the quarterly results may not equal full year results due to rounding.

[2] The prior quarter results have been restated to conform to the current quarter presentation. Specifically, results related to properties sold or held for sale have been reclassified into discontinued operations.