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 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
Beginning Balance, Shares at Dec. 31, 2015		4,000,000	174,688,568	745,686
Net income	52,871						52,446			425
Other comprehensive income (loss)	(9,946)							(9,860)		(86)
Issuance of partnership interest in consolidated real estate venture	73									73
Conversion of LP Units to Common Shares			$ 1		874					(875)
Conversion of LP Units to Common Shares, Shares			55,303
Share-based compensation activity	4,934		$ 4		4,890		40
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
Adjustment to Non-controlling Interest					(290)					290
Preferred Share distributions	(5,175)								(5,175)
Distributions declared	(83,227)								(82,523)	(704)
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
Ending Balance, Shares at Sep. 30, 2016		4,000,000	175,140,760	899,457
Beginning 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
Beginning Balance, Shares at Dec. 31, 2016		4,000,000	175,140,760	899,457
Net income	48,015						47,631			384
Other comprehensive income (loss)	846							839		7
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	(46)									(46)
Equity issuance costs	(499)				(499)
Bonus share issuance	110				110
Bonus share issuance, Shares			6,752
Share-based compensation activity	5,907		$ 3		5,900		4
Share-based compensation activity, Shares			333,127	39,870
Share Issuance from/(to) Deferred Compensation Plan	(48)			$ 406	(48)	(406)
Share Issuance from/(to) Deferred Compensation Plan, Shares			(1,718)	61,639
Share Choice Plan Issuance, Shares			(1,423)
Adjustment to Non-controlling Interest					(42)					42
Preferred Share distributions	(2,032)								(2,032)
Preferred Share redemption charges	(3,181)								(3,181)
Distributions declared	(85,174)								(84,463)	(711)
Ending Balance at Sep. 30, 2017	$ 1,750,570		$ 1,755	$ 14,090	$ 3,167,481	$ (14,090)	$ 586,954	$ (906)	$ (2,021,568)	$ 16,854
Ending Balance, Shares at Sep. 30, 2017			175,477,498	1,000,966