Condensed Consolidated Statements Of Equity (USD $) In Thousands, unless otherwise specified	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	41,073			40,036			1,037
Other comprehensive income	15					15
Net share awards	5,056		(5,287)		10,343
Employee share purchase plan	498		168		330
Common share options exercised	352		308		44
Change in classification of deferred compensation plan	606		606
Change in redemption value of non-qualified share awards	(11,352)			(11,352)
Diversification of share awards within deferred compensation plan	2,428		1,770	658
Cash distributions declared to equity holders	(59,750)			(58,496)			(1,254)
Other	(2)	(1)	(1)
Ending balance at Mar. 31, 2014	2,739,105	966	3,593,633	(523,321)	(399,510)	(1,091)	68,428
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