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		108,667			107,074		1,593
Issuance of common shares, net		17,880	15	17,865
Shares issued under benefit plans, net		2,215	26	2,189
Dividends paid - common shares	[1]	(45,378)			(45,378)
Distributions to noncontrolling interests		(2,139)					(2,139)
Other comprehensive loss		(3,665)				(3,665)
Ending Balance at Mar. 31, 2016		1,622,590	3,785	1,636,296	(161,184)	(11,309)	155,002
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		36,396			30,826		5,570
Shares issued under benefit plans, net		5,665	10	5,655
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]	(49,404)			(49,404)
Distributions to noncontrolling interests		(8,651)					(8,651)
Other comprehensive loss		1,203				1,203
Ending Balance at Mar. 31, 2017		$ 1,746,863	$ 3,895	$ 1,769,133	$ (196,844)	$ (7,958)	$ 178,637

[1] Common dividend per share was $.385 and $.365 for the three months ended March 31, 2017 and 2016, respectively.