Consolidated Statements of Beneficiaries' Equity - USD ($) $ in Thousands	Total	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, Shares at Dec. 31, 2013		4,000,000	156,731,993		312,280
Beginning Balance at Dec. 31, 2013	$ 1,921,435	$ 40	$ 1,566	$ 2,971,596	$ 5,407	$ (5,407)	$ 522,528	$ (2,995)	$ (1,592,515)	$ 21,215
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	8,811						8,754			57
Other comprehensive loss	335							337		(2)
Issuance of Common Shares of Beneficial Interest, Shares			21,850,000
Issuance of Common Shares of Beneficial Interest	335,398		$ 219	335,179
Conversion of LP Units to Common Shares, Shares			41,734
Conversion of LP Units to Common Shares	0			655						(655)
Equity Issuance Costs	(452)			(452)
Share-based compensation activity, Shares			279,913
Share-based compensation activity	4,614		$ 5	4,597			12
Share Issuance from/to Deferred Compensation Plan, Shares			78,729		72,256
Share issuance from/to Deferred Compensation Plan	(90)			(90)	$ 812	(812)
Adjustment to non-controlling interest	0			(1,172)						1,172
Preferred Share distributions	(5,175)								(5,175)
Distributions declared	(74,986)								(74,198)	(788)
Ending Balance, Shares at Sep. 30, 2014		4,000,000	178,982,369		384,536
Ending Balance at Sep. 30, 2014	2,189,890	$ 40	$ 1,790	3,310,313	$ 6,219	(6,219)	531,294	(2,658)	(1,671,888)	20,999
Beginning Balance, Shares at Dec. 31, 2014		4,000,000	179,293,160		384,536
Beginning Balance at Dec. 31, 2014	2,159,326	$ 40	$ 1,793	3,314,693	$ 6,219	(6,219)	529,487	(4,607)	(1,700,579)	18,499
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	31,960						31,739			221
Other comprehensive loss	$ (3,917)							(3,883)		(34)
Repurchase and retirement of Common Shares of Beneficial Interest, Shares	(4,701,302)		(4,701,302)
Repurchase and retirement of Common Shares of Beneficial Interest	$ (60,817)		$ (47)	(60,770)
Issuance of partnership interest in joint venture	1,025									1,025
Bonus share issuance, Shares			8,447
Bonus share issuance	125			125
Equity Issuance Costs	(100)			(100)
Share-based compensation activity, Shares			509,675		280,011
Share-based compensation activity	4,043		$ 6	4,036			1
Share Issuance from/to Deferred Compensation Plan, Shares			86,723		81,139
Share issuance from/to Deferred Compensation Plan	(2)			(2)	$ 5,699	(5,699)
Adjustment to non-controlling interest	0			93						(93)
Preferred Share distributions	(5,175)								(5,175)
Distributions declared	(81,311)								(80,620)	(691)
Ending Balance, Shares at Sep. 30, 2015		4,000,000	175,196,703		745,686
Ending Balance at Sep. 30, 2015	$ 2,045,157	$ 40	$ 1,752	$ 3,258,075	$ 11,918	$ (11,918)	$ 561,227	$ (8,490)	$ (1,786,374)	$ 18,927