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	121,065			115,599			5,466
Other comprehensive income	37					37
Net share awards	5,718		(2,726)		8,444
Employee share purchase plan	66		65		$ 1
Common share options exercised	176		176
Change in classification of deferred compensation plan	(1,024)		(1,024)
Change in redemption value of non-qualified share awards	(3,764)			(3,764)
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	(64,256)			(62,928)			(1,328)
Purchase of non-controlling interests	(9,500)		(9,480)				(20)
Other	(38)		(38)
Ending balance at Mar. 31, 2015	2,939,909	976	3,656,105	(403,518)	$ (388,181)	(2,382)	76,909
Beginning 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	42,940			41,730			1,210
Other comprehensive income	32					32
Net share awards	6,453		(2,306)		8,759
Employee share purchase plan	65		63		2
Change in classification of deferred compensation plan	(2,376)		(2,376)
Change in redemption value of non-qualified share awards	(6,810)			(6,810)
Conversions of operating partnership units	0		134				(134)
Cash distributions declared to equity holders	(69,037)			(67,618)			(1,419)
Other	(8)	(1)	(7)
Ending balance at Mar. 31, 2016	$ 2,864,155	$ 975	$ 3,658,372	$ (491,275)	$ (378,032)	$ (1,881)	$ 75,996