Consolidated Statements of Beneficiaries' Equity - USD ($) $ in Thousands	Total	Preferred Stock	Common Stock	Deferred Compensation Payable in Common Shares	Additional Paid-in Capital	Common Shares in Grantor Trust	Cumulative Earnings	Accumulated Other Comprehensive Income (Loss)	Cumulative Distributions	Noncontrolling Interests
Beneficiaries' Equity, Beginning Balance at Dec. 31, 2016	$ 1,864,388	$ 40	$ 1,752	$ 13,684	$ 3,258,386	$ (13,684)	$ 520,914	$ (1,745)	$ (1,931,892)	$ 16,933
Beginning Balance (in shares) at Dec. 31, 2016		4,000,000	175,140,760	899,457
Net income	121,177						120,173			1,004
Other comprehensive income	4,178							4,144		34
Issuance of Common Shares of Beneficial Interest	51,225		$ 29		51,196
Issuance of Common Shares of Beneficial Interest (in shares)			2,858,991
Redemption of Preferred Shares	(96,850)	$ (40)			(96,810)
Redemption of Preferred Shares (in shares)		(4,000,000)
Issuance of partnership interest in consolidated real estate venture	85									85
Distributions from consolidated real estate venture	(48)									(48)
Equity issuance costs	(499)				(499)
Bonus share issuance	110				110
Bonus share issuance (in shares)			6,752
Share Choice Plan Issuance (in shares)			(1,423)
Preferred Share distributions	(2,032)								(2,032)
Preferred Share redemption charges	$ (3,181)								(3,181)
Repurchase and retirement of Common Shares of Beneficial Interest (in shares)	0
Share-based compensation activity	$ 6,698		$ 3		6,689		6
Share-based compensation activity (in shares)			333,127	39,870
Share Issuance from/(to) Deferred Compensation Plan	(768)			$ (1,239)	(768)	1,239
Share Issuance from/(to) Deferred Compensation Plan (in shares)			(52,971)	(44,591)
Reallocation of Noncontrolling Interest					(227)					227
Distributions declared	(117,613)								(116,636)	(977)
Beneficiaries' Equity, Ending Balance at Dec. 31, 2017	1,826,870	$ 0	$ 1,784	$ 12,445	3,218,077	(12,445)	641,093	2,399	(2,053,741)	17,258
Ending Balance (in shares) at Dec. 31, 2017		0	178,285,236	894,736
Net income	135,472						134,518			954
Other comprehensive income	2,669							2,630		39
Issuance of Common Shares of Beneficial Interest	416				416
Issuance of Common Shares of Beneficial Interest (in shares)			23,311
Issuance of partnership interest in consolidated real estate venture	16									16
Distributions from consolidated real estate venture	(94)									(94)
Share Choice Plan Issuance (in shares)			(1,285)
Preferred Share distributions	0
Repurchase and retirement of Common Shares of Beneficial Interest	$ (21,858)		$ (17)		(21,841)
Repurchase and retirement of Common Shares of Beneficial Interest (in shares)	(1,729,278)		(1,729,278)
Redemption of LP Units	$ (7,043)									(7,043)
Share-based compensation activity	5,845		$ 2		5,829		14
Share-based compensation activity (in shares)			196,151
Share Issuance from/(to) Deferred Compensation Plan	(111)		$ 1	$ 1,576	(112)	(1,576)
Share Issuance from/(to) Deferred Compensation Plan (in shares)			99,189	82,384
Reallocation of Noncontrolling Interest	0				(2,057)					2,057
Distributions declared	(131,154)								(130,168)	(986)
Beneficiaries' Equity, Ending Balance at Dec. 31, 2018	1,811,028	$ 0	$ 1,770	$ 14,021	3,200,312	(14,021)	775,625	5,029	(2,183,909)	12,201
Ending Balance (in shares) at Dec. 31, 2018		0	176,873,324	977,120
Net income	34,529						34,267			262
Other comprehensive income	(7,440)							(7,399)		(41)
Issuance of partnership interest in consolidated real estate venture	27									27
Preferred Share distributions	0
Repurchase and retirement of Common Shares of Beneficial Interest	$ (17,281)		$ (13)		(17,268)
Repurchase and retirement of Common Shares of Beneficial Interest (in shares)	(1,337,169)		(1,337,169)
Purchase of partnership interest in consolidated real estate venture	$ (2,180)				(983)					(1,197)
Redemption of LP Units					16					(16)
Redemption of LP Units (in shares)			1,245
Share-based compensation activity	10,027		$ 9		10,018
Share-based compensation activity (in shares)			845,210	41,342
Share Issuance from/(to) Deferred Compensation Plan				$ 2,195		(2,195)
Share Issuance from/(to) Deferred Compensation Plan (in shares)			97,485	87,080
Reallocation of Noncontrolling Interest					63					(63)
Distributions declared	(135,071)								(134,324)	(747)
Beneficiaries' Equity, Ending Balance at Dec. 31, 2019	$ 1,688,303	$ 0	$ 1,766	$ 16,216	$ 3,192,158	$ (16,216)	$ 804,556	$ (2,370)	$ (2,318,233)	$ 10,426
Ending Balance (in shares) at Dec. 31, 2019		0	176,480,095	1,105,542