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		146,318			142,890		3,428
Issuance of common shares, net		123,789	99	123,690
Shares issued under benefit plans, net		4,886	35	4,851
Change in classification of deferred compensation plan		(37,488)		(37,488)
Change in redemption value of deferred compensation plan		(14,471)			(14,471)
Diversification of share awards within deferred compensation plan		3,819		3,819
Dividends paid - common shares	[1]	(91,675)			(91,675)
Distributions to noncontrolling interests		(3,493)					(3,493)
Other comprehensive income (loss)		(6,989)				(6,989)
Ending Balance at Jun. 30, 2016		1,669,706	3,878	1,711,114	(186,136)	(14,633)	155,483
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		105,589			94,678		10,911
Shares issued under benefit plans, net		7,176	11	7,165
Change in redemption value of deferred compensation plan		(619)			(619)
Dividends paid - common shares	[1]	(98,844)			(98,844)
Distributions to noncontrolling interests		(15,799)					(15,799)
Other comprehensive income (loss)		1,260				1,260
Change in classification of deferred compensation plan (see Note 1)		45,377		45,377
Other, net		(931)		(228)			(703)
Ending Balance at Jun. 30, 2017		$ 1,760,105	$ 3,896	$ 1,770,415	$ (182,432)	$ (7,901)	$ 176,127

[1] Common dividend per share was $.77 and $.73 for the six months ended June 30, 2017 and 2016, respectively.