Condensed Consolidated Statements of Equity (Unaudited) - 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 Dividend Paid	Net Income Less Than Accumulated Dividends Dividend Declared	Net Income Less Than Accumulated Dividends	Accumulated Other Comprehensive Loss [Member]	Noncontrolling Interests	Cumulative Effect, Period of Adoption, Adjustment [Member]	Dividend Paid	Dividend Declared	Total
Increase (Decrease) in Equity [Roll Forward]
Cumulative effect adjustment of accounting standards	$ 3,893	$ 1,766,993				$ (186,431)	$ (10,549)	$ 176,793				$ 1,750,699
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
Cumulative effect adjustment of accounting standards	3,903	1,777,089				(187,581)	(10,480)	177,503				1,760,434
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												245,283
Cumulative effect adjustment of accounting standards	3,904	1,778,828				(98,681)	(10,320)	178,746				1,852,477
Other comprehensive income												229
Ending Balance at Sep. 30, 2019	3,904	1,778,828				(98,681)	(10,320)	178,746				1,852,477
Increase (Decrease) in Equity [Roll Forward]
Cumulative effect adjustment of accounting standards	3,903	1,777,089				(187,581)	(10,480)	177,503				1,760,434
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
Cumulative effect adjustment of accounting standards	3,904	1,778,320				(154,597)	(10,402)	177,625				1,794,850
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
Increase (Decrease) in Equity [Roll Forward]
Cumulative effect adjustment of accounting standards	3,904	1,778,320				(154,597)	(10,402)	177,625				1,794,850
Net Income						106,742		1,767				108,509
Shares issued under benefit plans, net		508										508
Cumulative effect adjustment of accounting standards	3,904	1,778,828				(98,681)	(10,320)	178,746				1,852,477
Dividends paid - common shares				(50,826)						(50,826)
Distributions to noncontrolling interests								(646)				(646)
Other comprehensive income							82					82
Ending Balance at Sep. 30, 2019	3,904	1,778,828				(98,681)	(10,320)	178,746				1,852,477
Increase (Decrease) in Equity [Roll Forward]
Cumulative effect adjustment of accounting standards	3,904	1,778,828				(98,681)	(10,320)	178,746				1,852,477
Cumulative effect adjustment of 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
Cumulative effect adjustment of accounting standards	3,890	1,767,559	$ (711)			(73,317)	(11,207)	179,320	$ (711)			1,866,245
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												94,086
Cumulative effect adjustment of accounting standards	3,891	1,769,115				(106,051)	(11,154)	182,028				1,876,160
Other comprehensive income												129
Ending Balance at Sep. 30, 2020	3,891	1,769,115				(106,051)	(11,154)	182,028				1,837,829
Increase (Decrease) in Equity [Roll Forward]
Cumulative effect adjustment of 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
Cumulative effect adjustment of accounting standards	3,890	1,767,972	$ (711)			(85,008)	(11,158)	180,036	$ (711)			1,855,732
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 accounting standards	3,890	1,767,972				(85,008)	(11,158)	180,036				1,855,732
Net Income						25,089		2,372				27,461
Shares issued under benefit plans, net	1	1,143										1,144
Cumulative effect adjustment of accounting standards	3,891	1,769,115				(106,051)	(11,154)	182,028				1,855,732
Dividends paid - common shares				$ (23,066)	$ (23,066)					$ (23,066)	$ (23,066)
Distributions to noncontrolling interests								(380)				(380)
Other comprehensive income							4					4
Ending Balance at Sep. 30, 2020	3,891	1,769,115				(106,051)	(11,154)	182,028				1,837,829
Increase (Decrease) in Equity [Roll Forward]
Cumulative effect adjustment of accounting standards	$ 3,891	$ 1,769,115				$ (106,051)	$ (11,154)	$ 182,028				$ 1,837,829