Consolidated Statements of Beneficiaries' Equity (USD $) In Thousands, except Share data	Total	Series E Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Deferred Compensation, Share-based Payments [Member]	Common Stock In Grantor Trust [Member]	Cumulative Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Cumulative Distributions [Member]	Noncontrolling Interest [Member]
Beginning Balance at Dec. 31, 2011	$ 1,889,696	$ 43	$ 1,424	$ 2,776,197	$ 5,631	$ (5,631)	$ 477,338	$ (6,079)	$ (1,392,332)	$ 33,105
Beginning Balance, shares at Dec. 31, 2011		4,300,000	142,690,755		292,646
Net income	33,227						32,804			423
Comprehensive Income (Loss)	(14,580)							(14,377)		(203)
Preferred Stock, Shares Issued		4,000,000
Preferred Stock, Value, Issued		40
Issuance of Preferred Shares	96,850			96,810
Equity Issuance Costs	(610)			(610)
Redemption of Preferred Shares, Shares		(2,000,000)
Redemption of Preferred Shares, Value	47,910	(20)		47,890
Conversion of LP Units to Common Shares, shares			20,464
Conversion of LP Units to Common Shares	(231)		1	149			(49)			(332)
Bonus Share Issuance, Shares			35,703
Bonus Share Issuance	387			387
Vesting of Restricted Shares, Shares			280,851		9,036
Vesting of Restricted Shares	(1,291)		3	(1,294)
Restricted Share Amortization	2,138			2,138
Vesting of Restricted Performance Units, Shares			249,797
Vesting of Restricted Performance Units	(1,329)		2	(1,331)
Restricted Performance Units Amortization	1,926			1,926
Exercise of Stock Options, Shares			165,159
Exercise of Stock Options	748		2	746
Adjustments to Additional Paid in Capital, Share-based Compensation, Stock Options, Requisite Service Period Recognition				1,121
Stock Option Amortization	1,121
Share Issuance from/to Deferred Compensation Plan, Shares			(5,564)		(10,937)
Share Issuance from/to Deferred Compensation Plan	0				(279)	279
Trustee Fees Paid in Shares, Shares			1,336
Trustee Fees Paid in Shares, Value	15			15
Adjustments to Non-controlling Interest	11			358						(347)
Preferred Share distributions	(7,832)								(7,832)
Preferred share redemption charge	(2,090)								(2,090)
Distributions declared ($0.15 per share)	(66,037)								(64,804)	(1,233)
Ending Balance at Sep. 30, 2012	1,884,209	63	1,432	2,828,722	5,352	(5,352)	510,093	(20,456)	(1,467,058)	31,413
Ending Balance, shares at Sep. 30, 2012		6,300,000	143,438,501		290,745
Beginning Balance at Dec. 31, 2012	1,773,516	40	1,434	2,780,194	5,352	(5,352)	479,734	(15,918)	(1,493,206)	21,238
Beginning Balance, shares at Dec. 31, 2012		4,000,000	143,538,733		290,745
Net income	22,187						21,992			195
Comprehensive Income (Loss)	10,210							10,093		117
Stock Issued During Period, Shares, New Issues			12,650,000
Issuance of Common Shares of Beneficial Interest, Value, New Issues	182,034		127	181,907
Preferred Stock, Shares Issued	4,000,000
Preferred Stock, Value, Issued	40
Equity Issuance Costs	(382)			(382)
Conversion of LP Units to Common Shares, shares			81,998
Conversion of LP Units to Common Shares	0		1	1,240						(1,241)
Bonus Share Issuance, Shares			27,918
Bonus Share Issuance	361			361
Vesting of Restricted Shares, Shares			147,495		7,050
Vesting of Restricted Shares	(902)		2	(904)
Restricted Share Amortization	2,565			2,565
Vesting of Restricted Performance Units, Shares			53,134		27,067
Vesting of Restricted Performance Units	(160)			(160)
Restricted Performance Units Amortization	3,718			3,718
Restricted Share Forfeitures	9						9
Exercise of Stock Options, Shares	207,941		207,941
Exercise of Stock Options	2,238		1	2,237
Adjustments to Additional Paid in Capital, Share-based Compensation, Stock Options, Requisite Service Period Recognition				546
Stock Option Amortization	546
Share Issuance from/to Deferred Compensation Plan, Shares			(4,838)		(11,839)
Share Issuance from/to Deferred Compensation Plan	0				79	(79)
Trustee Fees Paid in Shares, Shares			1,409
Trustee Fees Paid in Shares, Value	20			20
Adjustments to Non-controlling Interest	0			(766)						766
Preferred Share distributions	(5,175)								(5,175)
Preferred share redemption charge	0
Distributions declared ($0.15 per share)	(69,627)								(68,821)	(806)
Ending Balance at Sep. 30, 2013	$ 1,921,158	$ 40	$ 1,565	$ 2,970,576	$ 5,431	$ (5,431)	$ 501,735	$ (5,825)	$ (1,567,202)	$ 20,269
Ending Balance, shares at Sep. 30, 2013		4,000,000	156,703,790		313,023