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		36,396			30,826		5,570
Shares issued under benefit plans, net		5,665	10	5,655
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]	(49,404)			(49,404)
Distributions to noncontrolling interests		(8,651)					(8,651)
Other comprehensive income		1,203				1,203
Ending Balance at Mar. 31, 2017		1,746,863	3,895	1,769,133	(196,844)	(7,958)	178,637
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		148,969			146,824		2,145
Shares issued under benefit plans, net		5,346	7	5,339
Dividends paid - common shares	[1]	(50,836)			(50,836)
Distributions to noncontrolling interests		(884)					(884)
Other comprehensive income		(1,983)				(1,983)
Shares repurchased and cancelled		(8,108)	(9)	(8,099)
Contributions from noncontrolling interests		41					41
Ending Balance at Mar. 31, 2018		1,906,343	$ 3,895	$ 1,769,306	(35,580)	(9,694)	$ 178,416
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 $.395 and $.385 for the three months ended March 31, 2018 and 2017, respectively.