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] Cumulative Effect, Period of Adoption, Adjustment [Member]	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	$ 3,897	$ 1,772,066		$ (137,065)		$ (6,170)	$ 177,114		$ 1,809,842
Beginning Balance at Dec. 31, 2017	3,897	1,772,066		(137,065)		(6,170)	177,114		1,809,842
Increase (Decrease) in Equity [Roll Forward]
Net Income				327,601			17,742		345,343
Shares repurchased and cancelled	(20)	(18,544)							(18,564)
Shares issued under benefit plans, net	16	13,471							13,487
Cumulative effect adjustment of new accounting standards	3,893	1,766,993	$ 5,497	(186,431)	$ (1,541)	(10,549)	176,793	$ 3,956	1,750,699
Dividends paid - common shares				(382,464)					(382,464)
Distributions to noncontrolling interests							(19,155)		(19,155)
Contributions from noncontrolling interests							1,465		1,465
Other comprehensive income						(2,838)			(2,838)
Other, net							(373)		(373)
Ending Balance at Dec. 31, 2018	3,893	1,766,993	5,497	(186,431)	(1,541)	(10,549)	176,793	3,956	1,750,699
Increase (Decrease) in Equity [Roll Forward]
Cumulative effect adjustment of new accounting standards	3,893	1,766,993	5,497	(186,431)	(1,541)	(10,549)	176,793	3,956	1,750,699
Net Income				315,435			7,140		322,575
Shares issued under benefit plans, net	12	11,046							11,058
Cumulative effect adjustment of new accounting standards	3,905	1,779,986	5,497	(74,293)	$ (1,541)	(11,283)	177,845	3,956	1,750,699
Dividends paid - common shares				(203,297)					(203,297)
Distributions to noncontrolling interests							(6,782)		(6,782)
Contributions from noncontrolling interests							326		326
Other comprehensive income						(734)			(734)
Other, net		1,947					368		2,315
Ending 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]
Cumulative effect adjustment of new accounting standards	3,905	1,779,986		(74,293)		(11,283)	177,845		1,876,160
Net Income				112,149			6,810		118,959
Shares repurchased and cancelled	(50)	(32,057)							(32,107)
Shares issued under benefit plans, net	11	7,841							7,852
Cumulative effect adjustment of new accounting standards	3,866	1,755,770	(711)	(128,813)		(12,050)	181,729	(711)	1,876,160
Dividends paid - common shares				(165,958)					(165,958)
Distributions to noncontrolling interests							(4,076)		(4,076)
Contributions from noncontrolling interests							1,150		1,150
Other comprehensive income						(767)			(767)
Ending Balance at Dec. 31, 2020	3,866	1,755,770	(711)	(128,813)		(12,050)	181,729	(711)	1,800,502
Increase (Decrease) in Equity [Roll Forward]
Cumulative effect adjustment of new accounting standards	$ 3,866	$ 1,755,770	$ (711)	$ (128,813)		$ (12,050)	$ 181,729	$ (711)	$ 1,800,502