Consolidated Statements of Equity - USD ($) $ in Thousands	Total	Preferred Shares [Member]	Common Shares [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Distributions in Excess of Retained Earnings [Member]	Parent [Member]	Non-controlling Interests [Member]
Preferred Stock, Shares Outstanding at Dec. 31, 2014		14,000,000
Common Stock, Shares, Outstanding at Dec. 31, 2014			71,553,481
Beginning balance at Dec. 31, 2014	$ 1,782,411	$ 140	$ 716	$ 1,864,739	$ (341)	$ (84,163)	$ 1,781,091	$ 1,320
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares, net of offering costs, Value	(195)			(195)			(195)
Issuance of Common Shares for Board Of Trustees Compensation , Shares			8,084
Issuance of Common Shares for Board Of Trustees Compensation, Value	372			372			372
Repurchase of Common Stock, Shares			(84,835)
Repurchase of Common Shares, Value	(4,094)			(4,094)			(4,094)
Share-based Compensation, Shares			258,399
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures			$ 1
Share-based Compensation, Value	6,614			5,783			5,784	830
Distributions on Common Shares and Units	(68,230)					(68,010)	(68,010)	(220)
Distributions on Preferred Shares	(19,470)					(19,463)	(19,463)	(7)
Unrealized gain (loss) on derivative instruments	(10,723)				(10,723)		(10,723)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	72,001					71,753	71,753	248
Preferred Stock, Shares Outstanding at Sep. 30, 2015		14,000,000
Common Stock, Shares, Outstanding at Sep. 30, 2015			71,735,129
Ending balance at Sep. 30, 2015	$ 1,758,686	$ 140	$ 717	1,866,605	(11,064)	(99,883)	1,756,515	2,171
Preferred Stock, Shares Outstanding at Dec. 31, 2015	14,000,000	14,000,000
Common Stock, Shares, Outstanding at Dec. 31, 2015	71,735,129		71,735,129
Beginning balance at Dec. 31, 2015	$ 1,760,834	$ 140	$ 717	1,868,047	(4,750)	(105,765)	1,758,389	2,445
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Shares, New Issues		5,000,000	0
Issuance of shares, net of offering costs, Value	120,821	$ 50	$ 0	120,771			120,821
Stock Redeemed or Called During Period, Shares		(9,000,000)
Stock Redeemed or Called During Period, Value	(225,050)	$ (90)		(217,870)		(7,090)	(225,050)
Issuance of Common Shares for Board Of Trustees Compensation , Shares			21,407
Issuance of Common Shares for Board Of Trustees Compensation, Value	606			606			606
Repurchase of Common Stock, Shares			(88,510)
Repurchase of Common Shares, Value	(2,496)		$ (1)	(2,495)			(2,496)
Share-based Compensation, Shares			254,878
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures			$ 3
Share-based Compensation, Value	6,185			5,354			5,357	828
Distributions on Common Shares and Units	(83,146)					(82,877)	(82,877)	(269)
Distributions on Preferred Shares	(15,647)					(15,638)	(15,638)	(9)
Unrealized gain (loss) on derivative instruments	(13,112)				(13,112)		(13,112)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ 55,540					55,346	55,346	194
Preferred Stock, Shares Outstanding at Sep. 30, 2016	10,000,000	10,000,000
Common Stock, Shares, Outstanding at Sep. 30, 2016	71,922,904		71,922,904
Ending balance at Sep. 30, 2016	$ 1,604,535	$ 100	$ 719	$ 1,774,413	$ (17,862)	$ (156,024)	$ 1,601,346	$ 3,189