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)/income	Non-controlling interests
Beginning balance at Dec. 31, 2016	$ 3,095,553	$ 978	$ 3,678,277	$ (289,180)	$ (373,339)	$ (1,863)	$ 80,680
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	35,989			34,861			1,128
Other comprehensive income	34					34
Net share awards	7,884		468		7,416
Employee share purchase plan	78		78
Common share options exercised	77		77
Change in classification of deferred compensation plan	(4,234)		(4,234)
Change in redemption value of non-qualified share awards	3,925			3,925
Diversification of share awards within deferred compensation plan	1,642		954	688
Conversion of operating partnership units	0		117				(117)
Cash distributions declared to equity holders	(69,349)			(67,936)			(1,413)
Ending balance at Mar. 31, 2017	3,071,599	978	3,675,737	(317,642)	(365,923)	(1,829)	80,278
Beginning balance at Dec. 31, 2016	3,095,553	978	3,678,277	(289,180)	(373,339)	(1,863)	80,680
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Diversification of share awards within deferred compensation plan	23,200
Ending balance at Dec. 31, 2017	3,484,714	1,028	4,137,161	(368,703)	(364,066)	(57)	79,351
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	40,525			39,395			1,130
Other comprehensive income	3,636					3,636
Net share awards	7,677		45		7,632
Employee share purchase plan	74		74
Common share options exercised	41		41
Change in classification of deferred compensation plan	(5,250)		(5,250)
Change in redemption value of non-qualified share awards	6,306			6,306
Common shares repurchased	(253)				(253)
Cash distributions declared to equity holders	(75,044)			(73,594)			(1,450)
Other	(17)	(2)	(15)
Ending balance at Mar. 31, 2018	$ 3,462,409	$ 1,026	$ 4,132,056	$ (396,596)	$ (356,687)	$ 3,579	$ 79,031