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, 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	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)
Reallocation of Noncontrolling 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
Beginning 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
Beginning Balance, Shares at Dec. 31, 2017			178,285,236	894,736
Net income	57,841						57,335			506
Other comprehensive income	6,879							6,822		57
Issuance of Common Shares of Beneficial Interest	416				416
Issuance of Common Shares of Beneficial Interest, Shares			23,311
Issuance of partnership interest in consolidated real estate venture	11									11
Distributions from consolidated real estate venture	(54)									(54)
Share-based compensation activity	4,215		$ 2		4,207		6
Share-based compensation activity, Shares			196,151
Share Issuance from/(to) Deferred Compensation Plan	(111)		$ 1	$ 1,591	(112)	(1,591)
Share Issuance from/(to) Deferred Compensation Plan, Shares			101,060	87,569
Share Choice Plan Issuance, Shares			(1,285)
Reallocation of Noncontrolling interest					(3)					3
Preferred Share distributions	0
Distributions declared	(65,022)								(64,489)	(533)
Ending Balance at Jun. 30, 2018	$ 1,850,775		$ 1,787	$ 14,036	$ 3,223,072	$ (14,036)	$ 717,515	$ 9,221	$ (2,118,230)	$ 17,410
Ending Balance, Shares at Jun. 30, 2018			178,604,473	982,305