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 [Member]	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, 2018	3,893	1,766,993	(186,431)	(10,549)	176,793	1,750,699
Increase (Decrease) in Equity [Roll Forward]
Net Income						136,774
Other comprehensive income						147
Ending Balance at Jun. 30, 2019	3,904	1,778,320	(154,597)	(10,402)	177,625	1,794,850
Beginning Balance at Mar. 31, 2019	3,903	1,777,089	(187,581)	(10,480)	177,503	1,760,434
Increase (Decrease) in Equity [Roll Forward]
Net Income			83,809		1,711	85,520
Shares issued under benefit plans, net	1	1,231				1,232
Dividends paid - common shares			(50,825)			(50,825)
Distributions to noncontrolling interests					(1,589)	(1,589)
Other comprehensive income				78		78
Ending Balance at Jun. 30, 2019	3,904	1,778,320	(154,597)	(10,402)	177,625	1,794,850
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
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						66,625
Other comprehensive income						125
Ending Balance at Jun. 30, 2020	3,890	1,767,972	(85,008)	(11,158)	180,036	1,855,732
Increase (Decrease) in Equity [Roll Forward]
Cumulative effect adjustment of new accounting standards			(711)			(711)
Beginning 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]
Net Income			11,368		1,009	12,377
Shares issued under benefit plans, net		413				413
Dividends paid - common shares			(23,059)			(23,059)
Distributions to noncontrolling interests					(293)	(293)
Other comprehensive income				49		49
Ending Balance at Jun. 30, 2020	$ 3,890	$ 1,767,972	$ (85,008)	$ (11,158)	$ 180,036	$ 1,855,732