Condensed Consolidated Statements of Equity (Unaudited) - USD ($) $ in Thousands	Common Shares of Beneficial Interest	Additional Paid-In Capital	Net Income Less Than Accumulated Dividends	Accumulated Other Comprehensive Loss	Noncontrolling Interests	Total
Beginning Balance at Dec. 31, 2018	$ 3,893	$ 1,766,993	$ (186,431)	$ (10,549)	$ 176,793	$ 1,750,699
Increase (Decrease) in Equity [Roll Forward]
Net Income			49,666		1,588	51,254
Shares issued under benefit plans, net	10	8,141				8,151
Dividends paid - common shares			(50,816)			(50,816)
Distributions to noncontrolling interests					(1,572)	(1,572)
Contributions from noncontrolling interests					326	326
Other comprehensive income				69		69
Other, net		1,955			368	2,323
Ending Balance at Mar. 31, 2019	3,903	1,777,089	(187,581)	(10,480)	177,503	1,760,434
Beginning Balance at Dec. 31, 2019	3,905	1,779,986	(74,293)	(11,283)	177,845	1,876,160
Increase (Decrease) in Equity [Roll Forward]
Net Income			52,622		1,626	54,248
Shares repurchased and cancelled	(25)	(18,194)				(18,219)
Shares issued under benefit plans, net	10	5,767				5,777
Dividends paid - common shares			(50,935)			(50,935)
Distributions to noncontrolling interests					(1,301)	(1,301)
Contributions from noncontrolling interests					1,150	1,150
Other comprehensive income				76		76
Ending Balance at Mar. 31, 2020	$ 3,890	$ 1,767,559	(73,317)	$ (11,207)	$ 179,320	1,866,245
Increase (Decrease) in Equity [Roll Forward]
Cumulative effect adjustment of new accounting standards			$ (711)			$ (711)