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 Loss [Member]	Cumulative Distributions [Member]	Noncontrolling Interest [Member]
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	$ 6,219	$ 3,314,693	$ (6,219)	$ 529,487	$ (4,607)	$ (1,700,579)	$ 18,499
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)			$ 5,699	(2)	(5,699)
Adjustment to Non-controlling Interest	0				93					(93)
Distribution to preferred shareholders	(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	$ 11,918	3,258,075	(11,918)	561,227	(8,490)	(1,786,374)	18,927
Beginning Balance, Shares at Dec. 31, 2015		4,000,000	174,688,568	745,686
Beginning 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
Net income	52,871						52,446			425
Other comprehensive loss	$ (9,946)							(9,860)		(86)
Repurchase and retirement of Common Shares of Beneficial Interest, Shares	0
Repurchase and retirement of Common Shares of Beneficial Interest	$ (67,300)
Issuance of partnership interest in consolidated real estate venture	73									73
Conversion of LP Units to Common Shares, Shares			55,303
Conversion of LP Units to Common Shares	0		$ 1		874					(875)
Share-based compensation activity, Shares			405,200
Share-based compensation activity	4,934		$ 4		4,890		40
Share Issuance from/to Deferred Compensation Plan, Shares			(8,311)	153,771
Share Issuance from/(to) Deferred Compensation Plan	(47)			$ 1,766	(47)	(1,766)
Adjustment to Non-controlling Interest	0				(290)					290
Distribution to preferred shareholders	(5,175)								(5,175)
Distributions declared	(83,227)								(82,523)	(704)
Ending Balance, Shares at Sep. 30, 2016		4,000,000	175,140,760	899,457
Ending Balance at Sep. 30, 2016	$ 1,911,574	$ 40	$ 1,752	$ 13,684	$ 3,258,049	$ (13,684)	$ 551,572	$ (15,052)	$ (1,902,076)	$ 17,289