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, 2016		$ 1,716,896	$ 3,885	$ 1,718,101	$ (177,647)	$ (9,161)	$ 181,718
Increase (Decrease) in Equity [Roll Forward]
Net Income		180,062			167,307		12,755
Shares issued under benefit plans, net		7,778	11	7,767
Change in classification of deferred compensation plan		45,377		45,377
Change in redemption value of deferred compensation plan		(619)			(619)
Dividends paid - common shares	[1]	(148,286)			(148,286)
Distributions to noncontrolling interests		(16,752)					(16,752)
Other comprehensive income (loss)		1,834				1,834
Other, net		(931)		(228)			(703)
Ending Balance at Sep. 30, 2017		1,785,359	3,896	1,771,017	(159,245)	(7,327)	177,018
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		282,114			268,094		14,020
Shares issued under benefit plans, net		13,022	16	13,006
Dividends paid - common shares	[1]	(152,110)			(152,110)
Distributions to noncontrolling interests		(15,063)					(15,063)
Other comprehensive income (loss)		(1,796)				(1,796)
Other, net		(4)					(4)
Shares repurchased and cancelled		(18,564)	(20)	(18,544)
Contributions from noncontrolling interests		1,324					1,324
Ending Balance at Sep. 30, 2018		1,922,721	$ 3,893	$ 1,766,528	(15,584)	(9,507)	$ 177,391
Increase (Decrease) in Equity [Roll Forward]
Cumulative effect adjustment of new accounting standards (see Note 2)		$ 3,956			$ 5,497	$ (1,541)

[1] Common dividend per share was $1.19 and $1.16 for the nine months ended September 30, 2018 and 2017, respectively.