Condensed Consolidated Statements Of Equity - USD ($) $ in Thousands	Total	Common shares of beneficial interest	Additional paid-in capital	Distributions in excess of net income	Treasury shares, at cost	Accumulated other comprehensive loss	Non-controlling interests
Beginning balance at Dec. 31, 2014	$ 2,888,409	$ 976	$ 3,667,448	$ (453,777)	$ (396,626)	$ (2,419)	$ 72,807
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	196,460			188,722			7,738
Other comprehensive income	112					112
Net share awards	17,055		7,792		9,263
Employee share purchase plan	583		334		249
Common share options exercised	176		176
Change in classification of deferred compensation plan	(7,924)		(7,924)
Change in redemption value of non-qualified share awards	184			184
Diversification of share awards within deferred compensation plan	3,558		2,135	1,423
Conversions of operating partnership units	0		86				(86)
Cash distributions declared to equity holders	(192,792)			(188,809)			(3,983)
Purchase of non-controlling interests	(9,500)		(9,480)				(20)
Other	(85)		(85)
Ending balance at Sep. 30, 2015	2,896,236	976	3,660,482	(452,257)	(387,114)	(2,307)	76,456
Beginning balance at Dec. 31, 2014	2,888,409	976	3,667,448	(453,777)	(396,626)	(2,419)	72,807
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Diversification of share awards within deferred compensation plan	3,600
Ending balance at Dec. 31, 2015	2,892,896	976	3,662,864	(458,577)	(386,793)	(1,913)	76,339
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	796,146			778,930			17,216
Other comprehensive income	97					97
Net share awards	18,912		9,175		9,737
Employee share purchase plan	1,298		757		541
Common share options exercised	3,921		1,003		2,918
Change in classification of deferred compensation plan	(10,110)		(10,110)
Change in redemption value of non-qualified share awards	(8,154)			(8,154)
Diversification of share awards within deferred compensation plan	25,406		11,913	13,493
Conversions of operating partnership units	(80)		217				(297)
Cash distributions declared to equity holders	(599,293)			(587,016)			(12,277)
Other	(11)	2	(13)
Ending balance at Sep. 30, 2016	$ 3,121,028	$ 978	$ 3,675,806	$ (261,324)	$ (373,597)	$ (1,816)	$ 80,981