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 (loss)	16,138						15,973			165
Comprehensive Income (Loss)	(10,564)							(10,370)		(194)
Preferred Stock, Shares Issued		4,000,000
Preferred Stock, Value, Issued		40
Preferred Stock, Including Additional Paid in Capital, Net of Discount	96,850			96,810
Equity Issuance Costs	(610)			(610)
Stock Redeemed or Called During Period, Shares		(2,000,000)
Stock Redeemed or Called During Period, Value		(20)
Additional Paid in Capital, Preferred Stock	(47,910)			(47,890)
Conversion of LP Units to Common Shares, shares			20,464
Conversion of LP Units to Common Shares	(228)			149			(45)			(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	(1,292)		3	(1,295)
Restricted Share Amortization	1,426			1,426
Vesting of Restricted Performance Units, Shares			249,797
Vesting of Restricted Performance Units	(1,329)		3	(1,332)
Restricted Performance Units Amortization	1,205			1,205
Exercise of Stock Options, Shares			94,429
Exercise of Stock Options	275		1	274
Stock Option Amortization	747			747
Share Issuance from/to Deferred Compensation Plan, Shares			(5,389)		(8,560)
Share Issuance from/to Deferred Compensation Plan	0				(195)	195
Trustee Fees Paid in Shares, Shares			1,336
Trustee Fees Paid in Shares, Value	15			15
Adjustments to Non-controlling Interest	13			392						(379)
Preferred Share distributions	(5,047)								(5,047)
Redemption Premium	(2,090)								(2,090)
Distributions declared ($0.15 per share)	(43,953)								(43,193)	(760)
Ending Balance at Jun. 30, 2012	1,893,729	63	1,431	2,826,475	5,436	(5,436)	493,266	(16,449)	(1,442,662)	31,605
Ending Balance, shares at Jun. 30, 2012		6,300,000	143,367,946		293,122
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 (loss)	11,099						11,011			88
Comprehensive Income (Loss)	11,451							11,317		134
Issuance of Common Shares of Beneficial Interest, 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	(380)			(380)
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	1,703			1,703
Vesting of Restricted Performance Units, Shares			26,067
Vesting of Restricted Performance Units	(160)			(160)
Restricted Performance Units Amortization	2,411			2,411
Restricted Share Forfeitures	9						9
Exercise of Stock Options, Shares	166,620		166,620
Exercise of Stock Options	1,761		1	1,760
Stock Option Amortization	377			377
Share Issuance from/to Deferred Compensation Plan, Shares			22,404		17,958
Share Issuance from/to Deferred Compensation Plan	0				164	(164)
Trustee Fees Paid in Shares, Shares			1,409
Trustee Fees Paid in Shares, Value	20			20
Adjustments to Non-controlling Interest	0			(739)						739
Preferred Share distributions	(3,450)								(3,450)
Redemption Premium	0
Distributions declared ($0.15 per share)	(45,781)								(45,240)	(541)
Ending Balance at Jun. 30, 2013	1,934,069	40	1,565	2,967,790	5,516	(5,516)	490,754	(4,601)	(1,541,896)	20,417
Ending Balance, shares at Jun. 30, 2013		4,000,000	156,662,644		315,753
Beginning Balance at Mar. 31, 2013
Net income (loss)	7,180
Comprehensive Income (Loss)	9,557
Preferred Stock, Shares Issued		4,000,000
Preferred Stock, Value, Issued	40
Redemption Premium	0
Ending Balance at Jun. 30, 2013	$ 1,934,069	$ 40
Ending Balance, shares at Jun. 30, 2013		4,000,000