Consolidated Statements of Equity - USD ($) $ in Thousands	Common Shares of Beneficial Interest	Additional Paid-In Capital	Net Income Less Than Accumulated Dividends Cumulative Effect, Period of Adoption, Adjustment [Member]	Net Income Less Than Accumulated Dividends	Accumulated Other Comprehensive Loss [Member]	Noncontrolling Interests	Cumulative Effect, Period of Adoption, Adjustment [Member]	Total
Increase (Decrease) in Equity [Roll Forward]
Cumulative effect adjustment of new accounting standards	$ 3,905	$ 1,779,986		$ (74,293)	$ (11,283)	$ 177,845		$ 1,876,160
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	$ (711)	(73,317)	(11,207)	179,320	$ (711)	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	3,890	1,767,559	(711)	(73,317)	(11,207)	179,320	(711)	1,866,245
Beginning Balance at Mar. 31, 2020	3,890	1,767,559	$ (711)	(73,317)	(11,207)	179,320	$ (711)	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
Increase (Decrease) in Equity [Roll Forward]
Cumulative effect adjustment of new accounting standards	3,890	1,767,972		(85,008)	(11,158)	180,036		1,855,732
Cumulative effect adjustment of new accounting standards	3,866	1,755,770		(128,813)	(12,050)	181,729		1,800,502
Beginning Balance at Dec. 31, 2020	3,866	1,755,770		(128,813)	(12,050)	181,729		1,800,502
Increase (Decrease) in Equity [Roll Forward]
Net Income				28,037		1,842		29,879
Shares issued under benefit plans, net	10	6,116						6,126
Dividends paid - common shares				(38,288)				(38,288)
Distributions to noncontrolling interests						(2,258)		(2,258)
Other comprehensive income					42			42
Other, net		(55)				(527)		(582)
Ending Balance at Mar. 31, 2021	3,876	1,761,831		(139,064)	(12,008)	180,786		1,795,421
Beginning Balance at Dec. 31, 2020	3,866	1,755,770		(128,813)	(12,050)	181,729		1,800,502
Increase (Decrease) in Equity [Roll Forward]
Net Income								44,320
Other comprehensive income								103
Ending Balance at Jun. 30, 2021	3,876	1,763,163		(155,730)	(11,947)	181,101		1,780,463
Increase (Decrease) in Equity [Roll Forward]
Cumulative effect adjustment of new accounting standards	3,876	1,761,831		(139,064)	(12,008)	180,786		1,795,421
Beginning Balance at Mar. 31, 2021	3,876	1,761,831		(139,064)	(12,008)	180,786		1,795,421
Increase (Decrease) in Equity [Roll Forward]
Net Income				12,692		1,749		14,441
Shares issued under benefit plans, net		1,332						1,332
Dividends paid - common shares				(29,358)				(29,358)
Distributions to noncontrolling interests						(1,434)		(1,434)
Other comprehensive income					61			61
Ending Balance at Jun. 30, 2021	3,876	1,763,163		(155,730)	(11,947)	181,101		1,780,463
Increase (Decrease) in Equity [Roll Forward]
Cumulative effect adjustment of new accounting standards	$ 3,876	$ 1,763,163		$ (155,730)	$ (11,947)	$ 181,101		$ 1,780,463