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
Common Stock, Dividends, Per Share, Declared	$ 0.77
Beginning 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 (loss)	3,636					3,636
Common shares issued	0
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 ($0.80 per common share)	(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
Common Stock, Dividends, Per Share, Declared	$ 0.80
Beginning balance at Dec. 31, 2018	$ 3,385,104	1,031	4,154,763	(495,496)	(355,804)	6,929	73,681
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	39,721			38,613			1,108
Other comprehensive income (loss)	(6,313)					(6,313)
Common shares issued	328,374	34	328,340
Net share awards	7,283		1,133		6,150
Employee share purchase plan	73		74		(1)
Change in classification of deferred compensation plan (See Note 11)	52,674		43,311	9,363
Cash distributions declared to equity holders ($0.80 per common share)	(80,741)			(79,336)			(1,405)
Other	145	(1)	38				108
Ending balance at Mar. 31, 2019	$ 3,726,320	$ 1,064	$ 4,527,659	$ (526,856)	$ (349,655)	$ 616	$ 73,492