Consolidated Statements of Beneficiaries' Equity - USD ($) $ in Thousands	Total	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, 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	44,705					44,329			376
Other comprehensive income	5,008						4,966		42
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	15								15
Distributions from consolidated real estate ventures	(54)								(54)
Share-based compensation activity	3,076	$ 1		3,072		3
Share-based compensation activity, Shares		68,425
Share Issuance from/(to) Deferred Compensation Plan	$ 0		$ 1,061		(1,061)
Share Issuance from/(to) Deferred Compensation Plan, Shares		66,830	53,856
Share Choice Plan issuance	0	(1,285)
Reallocation of Noncontrolling interest	$ 0			(5)					5
Distributions declared	(32,525)							(32,259)	(266)
Ending Balance at Mar. 31, 2018	1,867,241	$ 1,785	$ 13,506	3,222,047	(13,506)	704,506	7,365	(2,086,000)	17,538
Ending Balance, Shares at Mar. 31, 2018		178,442,517	948,592
Beginning Balance at Dec. 31, 2018	1,832,573	$ 1,770	$ 14,021	3,200,850	(14,021)	796,513	5,029	(2,183,909)	12,320
Beginning Balance, Shares at Dec. 31, 2018		176,873,324	977,120
Cumulative effect of accounting change	(5,336)					(5,336)
Net income	4,066					4,009			57
Other comprehensive income	(2,483)						(2,469)		(14)
Repurchase and retirement of Common Shares of Beneficial Interest	$ (17,281)	$ (13)		(17,268)
Repurchase and retirement of Common Shares of Beneficial Interest, Shares	(1,337,169)	(1,337,169)
Issuance of partnership interest in consolidated real estate ventures	$ 23								23
Share-based compensation activity	3,677	$ 4		3,673
Share-based compensation activity, Shares		465,883	41,342
Share Issuance from/(to) Deferred Compensation Plan	0		$ 619		(619)
Share Issuance from/(to) Deferred Compensation Plan, Shares		(458)	(5,920)
Reallocation of Noncontrolling interest	0			57					(57)
Distributions declared	(33,747)							(33,560)	(187)
Ending Balance at Mar. 31, 2019	$ 1,781,492	$ 1,761	$ 14,640	$ 3,187,312	$ (14,640)	$ 795,186	$ 2,560	$ (2,217,469)	$ 12,142
Ending Balance, Shares at Mar. 31, 2019		176,001,580	1,012,542