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	44,987						44,620			367
Other comprehensive income (loss)	(12,686)							(12,577)		(109)
Issuance of partnership interest in consolidated real estate venture	54									54
Conversion of LP Units to Common Shares			$ 1		874					(875)
Conversion of LP Units to Common Shares, Shares			55,303
Share-based compensation activity	3,615		$ 3		3,575		37
Share-based compensation activity, Shares			365,414
Share Issuance from/(to) Deferred Compensation Plan	(46)			$ 1,826	(46)	(1,826)
Share Issuance from/(to) Deferred Compensation Plan, Shares			(8,252)	155,583
Adjustment to Non-controlling Interest					(290)					290
Preferred Share distributions	(3,450)								(3,450)
Distributions declared	(54,739)								(54,272)	(467)
Ending Balance at Jun. 30, 2016	1,929,826	$ 40	$ 1,751	$ 13,744	3,256,735	(13,744)	543,743	(17,769)	(1,872,100)	17,426
Ending Balance, Shares at Jun. 30, 2016		4,000,000	175,101,033	901,269
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	28,969						28,755			214
Other comprehensive income (loss)	220							217		3
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	33									33
Equity issuance costs	(491)				(491)
Bonus share issuance	110				110
Bonus share issuance, Shares			6,752
Share-based compensation activity	4,344		$ 2		4,338		4
Share-based compensation activity, Shares			245,444	39,870
Share Issuance from/(to) Deferred Compensation Plan	(48)			$ 423	(48)	(423)
Share Issuance from/(to) Deferred Compensation Plan, Shares			(1,718)	61,639
Share Choice Plan Issuance, Shares			(1,423)
Adjustment to Non-controlling Interest					(34)					34
Preferred Share distributions	(2,032)								(2,032)
Preferred Share redemption charges	(3,181)								(3,181)
Distributions declared	(56,788)								(56,314)	(474)
Ending Balance at Jun. 30, 2017	$ 1,757,723		$ 1,754	$ 14,107	$ 3,165,935	$ (14,107)	$ 568,078	$ (1,528)	$ (1,993,419)	$ 16,903
Ending Balance, Shares at Jun. 30, 2017			175,389,815	1,000,966