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
Cash distributions declared to equity holders per common share	$ 2.31
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	120,387			116,932			3,455
Other comprehensive income (loss)	14,088					14,088
Common shares issued	0
Net share awards	17,570		9,341		8,229
Employee share purchase plan	720		455		265
Common share options exercised	41		41
Change in classification of deferred compensation plan	(13,547)		(13,547)
Change in redemption value of non-qualified share awards	(2,048)			(2,048)
Diversification of share awards within deferred compensation plan (Equity)	31,951		23,780	8,171
Conversion of operating partnership units	(14,415)		(9,781)				(4,634)
Common shares repurchased	(253)				(253)
Cash distributions declared to equity holders	(225,115)			(220,864)			(4,251)
Other	(170)	2	(172)
Ending balance at Sep. 30, 2018	$ 3,413,923	1,030	4,147,278	(466,512)	(355,825)	14,031	73,921
Cash distributions declared to equity holders per common share	$ 0.77
Beginning balance at Jun. 30, 2018	$ 3,428,604	1,027	4,132,404	(436,575)	(355,752)	8,794	78,706
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	39,990			38,866			1,124
Other comprehensive income (loss)	5,237					5,237
Net share awards	4,369		4,442		(73)
Employee share purchase plan	63		63
Change in classification of deferred compensation plan	(2,912)		(2,912)
Change in redemption value of non-qualified share awards	(3,024)			(3,024)
Diversification of share awards within deferred compensation plan (Equity)	31,000		23,143	7,857
Conversion of operating partnership units	(14,415)		(9,859)				(4,556)
Cash distributions declared to equity holders	(74,989)			(73,636)			(1,353)
Other	0	3	(3)
Ending balance at Sep. 30, 2018	$ 3,413,923	1,030	4,147,278	(466,512)	(355,825)	14,031	73,921
Cash distributions declared to equity holders per common share	$ 2.40
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	128,045			124,609			3,436
Other comprehensive income (loss)	(13,367)					(13,367)
Common shares issued	328,374	34	328,340
Net share awards	17,219		10,565		6,654
Employee share purchase plan	1,807		1,213		594
Change in classification of deferred compensation plan (See Note 11)	52,674		43,311	9,363
Conversion of operating partnership units	0		186				(186)
Cash distributions declared to equity holders	(242,299)			(238,091)			(4,208)
Other	224	0	44				180
Ending balance at Sep. 30, 2019	$ 3,657,781	1,065	4,538,422	(599,615)	(348,556)	(6,438)	72,903
Cash distributions declared to equity holders per common share	$ 0.80
Beginning balance at Jun. 30, 2019	$ 3,688,768	1,065	4,533,667	(563,834)	(348,480)	(6,795)	73,145
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	44,782			43,597			1,185
Other comprehensive income (loss)	357					357
Net share awards	4,549		4,625		(76)
Employee share purchase plan	57		57
Conversion of operating partnership units	0		71				(71)
Cash distributions declared to equity holders	(80,778)			(79,378)			(1,400)
Other	46		2				44
Ending balance at Sep. 30, 2019	$ 3,657,781	$ 1,065	$ 4,538,422	$ (599,615)	$ (348,556)	$ (6,438)	$ 72,903