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 (Loss) Attributable to Parent	113,591			113,591
Net Income (Loss) Attributable to Noncontrolling Interest	3,099						3,099
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	116,690
Other comprehensive income	(368)					(368)
Common shares issued	50,535	7	50,528
Net share awards	14,773		3,538		11,235
Employee share purchase plan	1,932		870		1,062
Common share options exercised	769	1	335		433
Change in classification of deferred compensation plan	(4,916)		(4,916)
Change in redemption value of non-qualified share awards	(12,936)			(12,936)
Diversification of share awards within deferred compensation plan	4,669		3,273	1,396
Conversions of operating partnership units			52				(52)
Cash distributions declared to equity holders	(179,784)			(176,026)			(3,758)
Other	0	(1)	1
Ending balance at Sep. 30, 2014	2,751,545	974	3,649,750	(568,142)	(397,497)	(1,474)	67,934
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 (Loss) Attributable to Parent	188,722			188,722
Net Income (Loss) Attributable to Noncontrolling Interest	7,738						7,738
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	196,460
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