Consolidated Statements Of Equity - USD ($) $ in Thousands	Total	Preferred Shares of Beneficial Interest	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, 2014	$ 1,638,943	$ 2	$ 3,700	$ 1,706,880	$ (212,960)	$ (12,436)	$ 153,757
Increase (Decrease) in Equity [Roll Forward]
Net Income	181,222				174,352		6,870
Redemption of preferred shares	(150,000)	(2)		(140,311)	(9,687)
Issuance of common shares, net	40,328		34	40,294
Shares issued under benefit plans, net	8,999		10	8,989
Shares issued in exchange for noncontrolling interests	0			111			(111)
Dividends paid – common shares	(170,755)				(170,755)
Dividends paid – preferred shares	(3,873)				(3,873)
Distributions to noncontrolling interests	(5,478)						(5,478)
Contributions from noncontrolling interests	1,318						1,318
Other comprehensive income (loss)	4,792					4,792
Other, net	(486)			279	43		(808)
Ending balance at Dec. 31, 2015	1,545,010	0	3,744	1,616,242	(222,880)	(7,644)	155,548
Increase (Decrease) in Equity [Roll Forward]
Net Income	276,831				238,933		37,898
Issuance of common shares, net	131,422		105	131,317
Shares issued under benefit plans, net	7,466		36	7,430
Dividends paid – common shares	(185,100)				(185,100)
Distributions to noncontrolling interests	(9,563)						(9,563)
Other comprehensive income (loss)	(1,517)					(1,517)
Other, net	(26)						(26)
Change in classification of deferred compensation plan	(39,977)			(39,977)
Change in redemption value of deferred compensation plan	(8,600)				(8,600)
Diversification of share awards within deferred compensation plan	3,819			3,819
Acquisition of noncontrolling interests	(2,869)			(730)			(2,139)
Ending balance at Dec. 31, 2016	1,716,896	0	3,885	1,718,101	(177,647)	(9,161)	181,718
Increase (Decrease) in Equity [Roll Forward]
Net Income	350,715				335,274		15,441
Shares issued under benefit plans, net	8,828		12	8,816
Dividends paid – common shares	(294,073)				(294,073)
Distributions to noncontrolling interests	(19,342)						(19,342)
Other comprehensive income (loss)	2,991					2,991
Other, net	(931)			(228)			(703)
Change in classification of deferred compensation plan	45,377			45,377
Change in redemption value of deferred compensation plan	(619)				(619)
Ending balance at Dec. 31, 2017	$ 1,809,842	$ 0	$ 3,897	$ 1,772,066	$ (137,065)	$ (6,170)	$ 177,114