Unaudited Quarterly Financial Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Information Disclosure [Abstract]
Total gross revenues	$ 95,533	[1]	$ 87,473	[1]	$ 82,750	[1]	$ 79,123	[1]	$ 79,570	$ 79,492	$ 77,474	$ 77,290	$ 344,879	$ 313,826	$ 305,350
Net income (loss)	(1,755)		175,289		5,626		5,478		14,016	(30,844)	(56,957)	(15,993)	184,638	(89,778)	(37,410)
Net income (loss) attributable to common shareholders	$ (7,039)		$ 168,950		$ (3,392)		$ (2,187)		$ 7,504	$ (37,048)	$ (50,539)	$ (23,638)	$ 180,316	$ (79,584)	$ (32,960)
Net income (loss) attributable to common shareholders - basic and diluted per share									$ 0.05	$ (0.24)	$ (0.33)	$ (0.16)
Earnings Per Share, Basic	$ (0.04)		$ 1.09		$ (0.02)		$ (0.01)						$ 0.99	$ (0.68)	$ (0.44)
Net income (loss) attributable to common shareholders (USD per share)	$ (0.04)		$ 0.96		$ (0.02)		$ (0.01)		$ 0.05	$ (0.24)	$ (0.33)	$ (0.16)	$ 0.93	$ (0.68)	$ (0.44)

[1] All periods have been adjusted to reflect the impact of properties sold during the years ended December 31, 2012 and 2011, and properties classified as held for sale, which are reflected in discontinued operations in the Consolidated Statements of Operations.