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, 2015		$ 1,545,010	$ 3,744	$ 1,616,242	$ (222,880)	$ (7,644)	$ 155,548
Increase (Decrease) in Equity [Roll Forward]
Net Income		207,655			194,791		12,864
Issuance of common shares, net		131,522	105	131,417
Shares issued under benefit plans, net		6,871	36	6,835
Change in classification of deferred compensation plan (see Note 1)		(38,787)		(38,787)
Change in redemption value of deferred compensation plan		(12,366)			(12,366)
Diversification of share awards within deferred compensation plan		3,819		3,819
Dividends paid - common shares	[1]	(138,354)			(138,354)
Distributions to noncontrolling interests		(6,252)					(6,252)
Other comprehensive income (loss)		(4,607)				(4,607)
Ending Balance at Sep. 30, 2016		1,694,511	3,885	1,719,526	(178,809)	(12,251)	162,160
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 (see Note 1)		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

[1] Common dividend per share was $1.16 and $1.10 for the nine months ended September 30, 2017 and 2016, respectively.