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, 2013	$ 2,760,181	$ 967	$ 3,596,069	$ (494,167)	$ (410,227)	$ (1,106)	$ 68,645
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	77,357			75,308			2,049
Other comprehensive income	29					29
Net share awards	9,880		(1,255)		11,135
Employee share purchase plan	886		555		331
Common share options exercised	647	1	359		287
Change in classification of deferred compensation plan	2,156		2,156
Change in redemption value of non-qualified share awards	14,819			14,819
Diversification of share awards within deferred compensation plan	2,428		1,770	658
Cash distributions declared to equity holders	(119,536)			(117,030)			(2,506)
Other	(28)	(1)	(27)
Ending balance at Jun. 30, 2014	2,714,869	967	3,595,315	(550,050)	(398,474)	(1,077)	68,188
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