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		105,589			94,678		10,911
Shares issued under benefit plans, net		7,176	11	7,165
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]	(98,844)			(98,844)
Distributions to noncontrolling interests		(15,799)					(15,799)
Other comprehensive income (loss)		1,260				1,260
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
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		228,840			225,113		3,727
Shares issued under benefit plans, net		7,444	9	7,435
Dividends paid - common shares	[1]	(101,423)			(101,423)
Distributions to noncontrolling interests		(4,593)					(4,593)
Other comprehensive income (loss)		(1,897)				(1,897)
Shares repurchased and cancelled		(18,564)	(20)	(18,544)
Contributions from noncontrolling interests		389					389
Ending Balance at Jun. 30, 2018		1,923,994	$ 3,886	$ 1,760,957	(7,878)	(9,608)	$ 176,637
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 $.79 and $.77 for the six months ended June 30, 2018 and 2017, respectively.