Condensed Consolidated Statements of Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Shares of Beneficial Interest	Additional Paid-In Capital	Net Income Less Than Accumulated Dividends	Accumulated Other Comprehensive Loss	Noncontrolling Interests
Beginning Balance at Dec. 31, 2017	$ 1,809,842	$ 3,897	$ 1,772,066	$ (137,065)	$ (6,170)	$ 177,114
Increase (Decrease) in Equity [Roll Forward]
Net Income	148,969			146,824		2,145
Shares issued under benefit plans, net	5,346	7	5,339
Dividends paid - common shares	(50,836)			(50,836)
Distributions to noncontrolling interests	(884)					(884)
Contributions from noncontrolling interests	41					41
Other comprehensive income (loss)	(1,983)				(1,983)
Shares repurchased and cancelled	(8,108)	(9)	(8,099)
Ending Balance at Mar. 31, 2018	1,906,343	3,895	1,769,306	(35,580)	(9,694)	178,416
Beginning Balance at Dec. 31, 2017	1,809,842	3,897	1,772,066	(137,065)	(6,170)	177,114
Increase (Decrease) in Equity [Roll Forward]
Net Income	228,840
Other comprehensive income (loss)	(1,897)
Ending Balance at Jun. 30, 2018	1,923,994	3,886	1,760,957	(7,878)	(9,608)	176,637
Beginning Balance at Mar. 31, 2018	1,906,343	3,895	1,769,306	(35,580)	(9,694)	178,416
Increase (Decrease) in Equity [Roll Forward]
Net Income	79,871			78,289		1,582
Shares issued under benefit plans, net	2,098	2	2,096
Dividends paid - common shares	(50,587)			(50,587)
Distributions to noncontrolling interests	(3,709)					(3,709)
Contributions from noncontrolling interests	348					348
Other comprehensive income (loss)	86				86
Shares repurchased and cancelled	(10,456)	(11)	(10,445)
Ending Balance at Jun. 30, 2018	1,923,994	3,886	1,760,957	(7,878)	(9,608)	176,637
Beginning Balance at Dec. 31, 2018	1,750,699	3,893	1,766,993	(186,431)	(10,549)	176,793
Increase (Decrease) in Equity [Roll Forward]
Net Income	51,254			49,666		1,588
Shares issued under benefit plans, net	8,151	10	8,141
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 (loss)	69				69
Other, net	2,323		1,955			368
Ending Balance at Mar. 31, 2019	1,760,434	3,903	1,777,089	(187,581)	(10,480)	177,503
Beginning Balance at Dec. 31, 2018	1,750,699	3,893	1,766,993	(186,431)	(10,549)	176,793
Increase (Decrease) in Equity [Roll Forward]
Net Income	136,774
Other comprehensive income (loss)	147
Ending Balance at Jun. 30, 2019	1,794,850	3,904	1,778,320	(154,597)	(10,402)	177,625
Beginning Balance at Mar. 31, 2019	1,760,434	3,903	1,777,089	(187,581)	(10,480)	177,503
Increase (Decrease) in Equity [Roll Forward]
Net Income	85,520			83,809		1,711
Shares issued under benefit plans, net	1,232	1	1,231
Dividends paid - common shares	(50,825)			(50,825)
Distributions to noncontrolling interests	(1,589)					(1,589)
Other comprehensive income (loss)	78				78
Ending Balance at Jun. 30, 2019	$ 1,794,850	$ 3,904	$ 1,778,320	$ (154,597)	$ (10,402)	$ 177,625