Consolidated Statements of Beneficiaries' Equity - USD ($) $ in Thousands	Total	Common Stock	Rabbi Trust/Deferred Compensation Shares	Additional Paid-in Capital	Common Shares in Grantor Trust	Cumulative Earnings	Accumulated Other Comprehensive Income (Loss)	Cumulative Distributions	Noncontrolling Interests	Cumulative effect of accounting change	Cumulative effect of accounting change Cumulative Earnings
Beginning balance (in shares) at Dec. 31, 2018		176,873,324	977,120
Beginning balance at Dec. 31, 2018	$ 1,811,028	$ 1,770	$ 14,021	$ 3,200,312	$ (14,021)	$ 775,625	$ 5,029	$ (2,183,909)	$ 12,201	$ (5,336)	$ (5,336)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	4,583					4,523			60
Other comprehensive income (loss)	(2,483)						(2,469)		(14)
Issuance of partnership interest in consolidated real estate ventures	22								22
Repurchase and retirement of Common Shares of Beneficial Interest (in shares)		(1,337,169)
Repurchase and retirement of Common Shares of Beneficial Interest	(17,281)	$ (13)		(17,268)
Share-based compensation activity (in shares)		465,883	41,342
Share-based compensation activity	3,677	$ 4		3,673
Share Issuance from/(to) Deferred Compensation Plan (in shares)		(458)	(5,920)
Share Issuance from/(to) Deferred Compensation Plan	0		$ 619		(619)
Reallocation of Noncontrolling interest	0			57					(57)
Distributions declared	(33,747)							(33,560)	(187)
Ending balance (in shares) at Mar. 31, 2019		176,001,580	1,012,542
Ending balance at Mar. 31, 2019	1,760,463	$ 1,761	$ 14,640	3,186,774	(14,640)	774,812	2,560	(2,217,469)	12,025
Beginning balance (in shares) at Dec. 31, 2018		176,873,324	977,120
Beginning balance at Dec. 31, 2018	1,811,028	$ 1,770	$ 14,021	3,200,312	(14,021)	775,625	5,029	(2,183,909)	12,201	$ (5,336)	$ (5,336)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	17,655
Other comprehensive income (loss)	$ (8,627)
Repurchase and retirement of Common Shares of Beneficial Interest (in shares)	(1,337,169)
Repurchase and retirement of Common Shares of Beneficial Interest	$ (17,300)
Ending balance (in shares) at Sep. 30, 2019		176,194,918	1,105,542
Ending balance at Sep. 30, 2019	1,702,437	$ 1,763	$ 16,216	3,189,350	(16,216)	787,789	(3,550)	(2,284,610)	11,695
Beginning balance (in shares) at Mar. 31, 2019		176,001,580	1,012,542
Beginning balance at Mar. 31, 2019	1,760,463	$ 1,761	$ 14,640	3,186,774	(14,640)	774,812	2,560	(2,217,469)	12,025
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	6,252					6,205			47
Other comprehensive income (loss)	(4,746)						(4,720)		(26)
Issuance of partnership interest in consolidated real estate ventures	3								3
Redemption of LP Units (in shares)		1,245
Redemption of LP Units	0			16					(16)
Share-based compensation activity (in shares)		94,150
Share-based compensation activity	1,451	$ 2		1,449
Share Issuance from/(to) Deferred Compensation Plan (in shares)		100,908	100,908
Share Issuance from/(to) Deferred Compensation Plan	0		$ 1,599		(1,599)
Distributions declared	(33,758)							(33,571)	(187)
Ending balance (in shares) at Jun. 30, 2019		176,197,883	1,113,450
Ending balance at Jun. 30, 2019	1,729,665	$ 1,763	$ 16,239	3,188,239	(16,239)	781,017	(2,160)	(2,251,040)	11,846
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	6,820					6,772			48
Other comprehensive income (loss)	(1,398)						(1,390)		(8)
Issuance of partnership interest in consolidated real estate ventures	2								2
Share-based compensation activity	1,105			1,105
Share Issuance from/(to) Deferred Compensation Plan (in shares)		(2,965)	(7,908)
Share Issuance from/(to) Deferred Compensation Plan	0		$ (23)		23
Reallocation of Noncontrolling interest	0			6					(6)
Distributions declared	(33,757)							(33,570)	(187)
Ending balance (in shares) at Sep. 30, 2019		176,194,918	1,105,542
Ending balance at Sep. 30, 2019	1,702,437	$ 1,763	$ 16,216	3,189,350	(16,216)	787,789	(3,550)	(2,284,610)	11,695
Beginning balance (in shares) at Dec. 31, 2019		176,480,095	1,105,542
Beginning balance at Dec. 31, 2019	1,688,303	$ 1,766	$ 16,216	3,192,158	(16,216)	804,556	(2,370)	(2,318,233)	10,426
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	8,087					8,022			65
Other comprehensive income (loss)	(7,869)						(7,825)		(44)
Repurchase and retirement of Common Shares of Beneficial Interest (in shares)		(5,644,200)
Repurchase and retirement of Common Shares of Beneficial Interest	(53,858)	$ (57)		(53,801)
Share-based compensation activity (in shares)		142,468	50,967
Share-based compensation activity	2,031	$ 1		2,030
Share Issuance from/(to) Deferred Compensation Plan (in shares)		(12,376)	(38,726)
Share Issuance from/(to) Deferred Compensation Plan	(193)		$ 796	(193)	(796)
Distributions declared	(32,687)							(32,500)	(187)
Ending balance (in shares) at Mar. 31, 2020		170,965,987	1,117,783
Ending balance at Mar. 31, 2020	1,603,814	$ 1,710	$ 17,012	3,140,194	(17,012)	812,578	(10,195)	(2,350,733)	10,260
Beginning balance (in shares) at Dec. 31, 2019		176,480,095	1,105,542
Beginning balance at Dec. 31, 2019	1,688,303	$ 1,766	$ 16,216	3,192,158	(16,216)	804,556	(2,370)	(2,318,233)	10,426
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	288,240
Other comprehensive income (loss)	$ (6,415)
Repurchase and retirement of Common Shares of Beneficial Interest (in shares)	(6,248,483)
Repurchase and retirement of Common Shares of Beneficial Interest	$ (60,000)
Ending balance (in shares) at Sep. 30, 2020		170,572,964	1,160,494
Ending balance at Sep. 30, 2020	1,815,782	$ 1,707	$ 17,516	3,136,851	(17,516)	1,091,095	(8,749)	(2,415,736)	10,614
Beginning balance (in shares) at Mar. 31, 2020		170,965,987	1,117,783
Beginning balance at Mar. 31, 2020	1,603,814	$ 1,710	$ 17,012	3,140,194	(17,012)	812,578	(10,195)	(2,350,733)	10,260
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	4,020					3,996			24
Other comprehensive income (loss)	226						225		1
Repurchase and retirement of Common Shares of Beneficial Interest (in shares)		(604,283)
Repurchase and retirement of Common Shares of Beneficial Interest	(6,141)	$ (5)		(6,136)
Share-based compensation activity (in shares)		166,628
Share-based compensation activity	1,668	$ 2		1,666
Share Issuance from/(to) Deferred Compensation Plan (in shares)		44,827	42,906
Share Issuance from/(to) Deferred Compensation Plan	(11)		$ 504	(11)	(504)
Reallocation of Noncontrolling interest	0			(123)					123
Distributions declared	(32,727)							(32,540)	(187)
Ending balance (in shares) at Jun. 30, 2020		170,573,159	1,160,689
Ending balance at Jun. 30, 2020	1,570,849	$ 1,707	$ 17,516	3,135,590	(17,516)	816,574	(9,970)	(2,383,273)	10,221
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	276,133					274,521			1,612
Other comprehensive income (loss)	1,228						1,221		7
Impact of deconsolidation of real estate venture	(1,017)								(1,017)
Distributions from consolidated real estate ventures	(22)								(22)
Share-based compensation activity	1,263			1,263
Share Issuance from/(to) Deferred Compensation Plan (in shares)		(195)	(195)
Share Issuance from/(to) Deferred Compensation Plan	(2)			(2)
Distributions declared	(32,650)							(32,463)	(187)
Ending balance (in shares) at Sep. 30, 2020		170,572,964	1,160,494
Ending balance at Sep. 30, 2020	$ 1,815,782	$ 1,707	$ 17,516	$ 3,136,851	$ (17,516)	$ 1,091,095	$ (8,749)	$ (2,415,736)	$ 10,614