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	48,015						47,631			384
Other comprehensive income	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 ventures	85									85
Distributions from consolidated real estate ventures	(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)
Reallocation of Noncontrolling 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
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	14,579						14,412			167
Other comprehensive income	7,906							7,840		66
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 ventures	16									16
Distributions from consolidated real estate ventures	(54)									(54)
Share-based compensation activity	4,972		$ 2		4,957		13
Share-based compensation activity, Shares			196,151
Share Issuance from/(to) Deferred Compensation Plan	(111)		$ 1	$ 1,576	(112)	(1,576)
Share Issuance from/(to) Deferred Compensation Plan, Shares			99,189	82,384
Share Choice Plan Issuance, Shares			(1,285)
Reallocation of Noncontrolling interest					(8)					8
Preferred Share distributions	0
Distributions declared	(97,521)								(96,722)	(799)
Ending Balance at Sep. 30, 2018	$ 1,776,803		$ 1,787	$ 14,021	$ 3,223,817	$ (14,021)	$ 674,599	$ 10,239	$ (2,150,463)	$ 16,824
Ending Balance, Shares at Sep. 30, 2018			178,602,602	977,120