Consolidated Statements of Equity (USD $) In Thousands, unless otherwise specified	Total	Total Shareholders' Equity [Member]	Preferred Shares of Beneficial Interest [Member]	Common Shares of Beneficial Interest [Member]	Treasury Shares [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Distributions In Excess of Retained Earnings [Member]	Total Noncontrolling Interests [Member]	Noncontrolling Interests in Consolidated Entities [Member]	Noncontrolling Interests of Common Units in Operating Partnership [Member]
Balance, Beginning of year at Dec. 31, 2013	$ 2,109,463	$ 2,103,391	$ 95	$ 1,039	$ (14)	$ 2,379,246	$ 4,603	$ (281,578)	$ 6,072	$ 18	$ 6,054
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares, net of offering costs	602	602	0	1	0	601	0	0	0	0	0
Repurchase of common shares into treasury	(1,201)	(1,201)	0	0	(1,201)	0	0	0	0	0	0
Deferred compensation, net	1,549	1,549	0	0	1,215	334	0	0	0	0	0
Adjustments to noncontrolling interests	0	10	0	0	0	10	0	0	(10)	0	(10)
Distributions on earned shares from share awards with market conditions	(75)	(75)	0	0	0	0	0	(75)	0	0	0
Distributions on common shares/units ($0.28 and $0.38 per share/unit, respectively)	(29,255)	(29,172)	0	0	0	0	0	(29,172)	(83)	0	(83)
Distributions on preferred shares	(4,107)	(4,107)	0	0	0	0	0	(4,107)	0	0	0
Net (loss) income	(4,871)	(4,865)	0	0	0	0	0	(4,865)	(6)	0	(6)
Other comprehensive loss:
Unrealized gain (loss) on interest rate derivative instruments	111	111	0	0	0	0	111	0	0	0	0
Reclassification adjustment for amounts recognized in net (loss) income	(1,083)	(1,080)	0	0	0	0	(1,080)	0	(3)	0	(3)
Balance, End of period at Mar. 31, 2014	2,071,133	2,065,163	95	1,040	0	2,380,191	3,634	(319,797)	5,970	18	5,952
Balance, Beginning of year at Dec. 31, 2014	2,448,386	2,441,709	72	1,127	(138)	2,673,888	748	(233,988)	6,677	17	6,660
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares, net of offering costs	669	669	0	0	0	669	0	0	0	0	0
Repurchase of common shares into treasury	(1,671)	(1,671)	0	0	(1,671)	0	0	0	0	0	0
Deferred compensation, net	1,854	1,854	0	0	1,809	45	0	0	0	0	0
Distributions on earned shares from share awards with market conditions	(334)	(334)	0	0	0	0	0	(334)	0	0	0
Distributions on common shares/units ($0.28 and $0.38 per share/unit, respectively)	(42,518)	(42,407)	0	0	0	0	0	(42,407)	(111)	0	(111)
Distributions on preferred shares	(3,042)	(3,042)	0	0	0	0	0	(3,042)	0	0	0
Net (loss) income	2,738	2,723	0	0	0	0	0	2,723	15	0	15
Other comprehensive loss:
Unrealized gain (loss) on interest rate derivative instruments	(4,398)	(4,386)	0	0	0	0	(4,386)	0	(12)	0	(12)
Reclassification adjustment for amounts recognized in net (loss) income	1,070	1,067	0	0	0	0	1,067	0	3	0	3
Balance, End of period at Mar. 31, 2015	$ 2,402,754	$ 2,396,182	$ 72	$ 1,127	$ 0	$ 2,674,602	$ (2,571)	$ (277,048)	$ 6,572	$ 17	$ 6,555