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	158,266			151,678			6,588
Other comprehensive income	74					74
Net share awards	11,374		2,169		9,205
Employee share purchase plan	528		279		249
Common share options exercised	176		176
Change in classification of deferred compensation plan	(4,678)		(4,678)
Change in redemption value of non-qualified share awards	1			1
Diversification of share awards within deferred compensation plan	3,020		1,668	1,352
Conversions of operating partnership units	0		16				(16)
Cash distributions declared to equity holders	(128,524)			(125,868)			(2,656)
Purchase of non-controlling interests	(9,500)		(9,480)				(20)
Other	(61)		(61)
Ending balance at Jun. 30, 2015	2,919,085	976	3,657,537	(426,614)	(387,172)	(2,345)	76,703
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	492,725			488,032			4,693
Other comprehensive income	65					65
Net share awards	13,201		3,519		9,682
Employee share purchase plan	1,223		682		541
Common share options exercised	3,572		916		2,656
Change in classification of deferred compensation plan	(6,688)		(6,688)
Change in redemption value of non-qualified share awards	(11,445)			(11,445)
Diversification of share awards within deferred compensation plan	25,017		11,701	13,316
Conversions of operating partnership units	0		255				(255)
Cash distributions declared to equity holders	(138,166)			(135,330)			(2,836)
Other	(10)	2	(12)
Ending balance at Jun. 30, 2016	$ 3,272,390	$ 978	$ 3,673,237	$ (104,004)	$ (373,914)	$ (1,848)	$ 77,941