Consolidated Statements of Beneficiaries' Equity - USD ($) $ in Thousands	Total	Preferred Stock [Member]	Common Stock [Member]	Deferred Compensation, Share-based Payments [Member]	Additional Paid-in Capital [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, 2014	$ 2,159,326	$ 40	$ 1,793	$ 6,219	$ 3,314,693	$ (6,219)	$ 529,487	$ (4,607)	$ (1,700,579)	$ 18,499
Beginning Balance, Shares at Dec. 31, 2014		4,000,000	179,293,160	384,536
Net income (loss)	(30,740)						(30,401)			(339)
Other comprehensive income (loss)	$ (590)							(585)		(5)
Issuance of Common Shares of Beneficial Interest, Shares	0
Repurchase and retirement of Common Shares of Beneficial Interest	$ (67,325)		$ (52)		(67,273)
Repurchase and retirement of Common Shares of Beneficial Interest, Shares	(5,209,437)		(5,209,437)
Issuance of partnership interest in joint venture	$ 1,025									1,025
Bonus share issuance	125				125
Bonus share issuance, Shares			8,447
Equity issuance costs	(105)				(105)
Share-based compensation activity	5,097		$ 6		5,091
Share-based compensation activity, Shares			509,675	280,011
Share Issuance from/(to) Deferred Compensation Plan	(2)			$ 5,699	(2)	(5,699)
Share Issuance from/(to) Deferred Compensation Plan, Shares			88,146	81,139
Share Choice Plan Issuance, Shares			(1,423)
Reallocation of Noncontrolling Interest					93					(93)
Preferred Share distributions	(6,900)								(6,900)
Distributions declared	(107,820)								(106,899)	(921)
Ending Balance at Dec. 31, 2015	1,952,091	$ 40	$ 1,747	$ 11,918	3,252,622	(11,918)	499,086	(5,192)	(1,814,378)	18,166
Ending Balance, Shares at Dec. 31, 2015		4,000,000	174,688,568	745,686
Net income (loss)	40,501						40,191			310
Other comprehensive income (loss)	$ 3,475							3,447		28
Issuance of Common Shares of Beneficial Interest, Shares	0
Issuance of partnership interest in consolidated real estate venture	$ 108									108
Conversion of LP Units to Common Shares			$ 1		874					(875)
Conversion of LP Units to Common Shares, Shares			55,303
Repurchase and retirement of Common Shares of Beneficial Interest, Shares	0
Share-based compensation activity	$ 5,764		$ 4		5,718		42
Share-based compensation activity, Shares			405,200
Share Issuance from/(to) Deferred Compensation Plan	(47)			$ 1,766	(47)	(1,766)
Share Issuance from/(to) Deferred Compensation Plan, Shares			(8,311)	153,771
Reallocation of Noncontrolling Interest					(297)					297
Preferred Share distributions	(6,900)								(6,900)
Distributions declared	(111,555)								(110,614)	(941)
Ending Balance at Dec. 31, 2016	1,883,437	$ 40	$ 1,752	$ 13,684	3,258,870	(13,684)	539,319	(1,745)	(1,931,892)	17,093
Ending Balance, Shares at Dec. 31, 2016		4,000,000	175,140,760	899,457
Net income (loss)	121,859						120,850			1,009
Other comprehensive income (loss)	4,178							4,144		34
Issuance of Common Shares of Beneficial Interest	51,225		$ 29		51,196
Issuance of Common Shares of Beneficial Interest, Shares			2,858,991
Redemption of Preferred Shares	(96,850)	$ (40)			(96,810)
Redemption of Preferred Shares, Shares		(4,000,000)
Issuance of partnership interest in consolidated real estate venture	85									85
Distributions from consolidated real estate venture	$ (48)									(48)
Repurchase and retirement of Common Shares of Beneficial Interest, Shares	0
Bonus share issuance	$ 110				110
Bonus share issuance, Shares			6,752
Equity issuance costs	(499)				(499)
Share-based compensation activity	6,697		$ 3		6,689		5
Share-based compensation activity, Shares			333,127	39,870
Share Issuance from/(to) Deferred Compensation Plan	(768)			$ (1,239)	(768)	1,239
Share Issuance from/(to) Deferred Compensation Plan, Shares			(52,971)	(44,591)
Share Choice Plan Issuance, Shares			(1,423)
Reallocation of Noncontrolling Interest					(224)					224
Preferred Share distributions	(2,032)								(2,032)
Preferred Share redemption charges	(3,181)								(3,181)
Distributions declared	(117,613)								(116,636)	(977)
Ending Balance at Dec. 31, 2017	$ 1,846,600		$ 1,784	$ 12,445	$ 3,218,564	$ (12,445)	$ 660,174	$ 2,399	$ (2,053,741)	$ 17,420
Ending Balance, Shares at Dec. 31, 2017			178,285,236	894,736