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]	Noncontrolling Interest [Member]
Preferred Stock, Shares Outstanding at Dec. 31, 2013		13,000,000
Common Stock, Shares, Outstanding at Dec. 31, 2013			63,709,628
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Dec. 31, 2013	$ 1,475,084	$ 130	$ 637	$ 1,541,138	$ 1,086	$ (69,652)	$ 1,473,339	$ 1,745
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares, net of offering costs, Shares		1,000,000	3,850,000
Issuance of shares, net of offering costs, Value	170,750	$ 10	$ 39	170,701			170,750
Issuance of Common Shares for Board Of Trustees Compensation , Shares			13,793
Issuance of Common Shares for Board Of Trustees Compensation, Value	421			421			421
Repurchase of Common Stock, Shares			(20,539)
Repurchase of Common Shares, Value	(632)			(632)			(632)
Share-based Compensation, Shares			62,047
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures			$ 0
Share-based Compensation, Value	8,237			6,225			6,225	2,012
Distributions on Common Shares and Units	(46,003)					(45,583)	(45,583)	(420)
Distributions on Preferred Shares	(18,598)					(18,591)	(18,591)	(7)
Unrealized gain (loss) on derivative instruments	(132)				(132)		(132)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	57,453					56,916	56,916	537
Preferred Stock, Shares Outstanding at Sep. 30, 2014		14,000,000
Common Stock, Shares, Outstanding at Sep. 30, 2014			67,614,929
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Sep. 30, 2014	$ 1,646,580	$ 140	$ 676	1,717,853	954	(76,910)	1,642,713	3,867
Preferred Stock, Shares Outstanding at Dec. 31, 2014	14,000,000	14,000,000
Common Stock, Shares, Outstanding at Dec. 31, 2014	71,553,481		71,553,481
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest 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, Shares		0	0
Issuance of shares, net of offering costs, Value	(195)	$ 0	$ 0	(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			$ 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	14,000,000
Common Stock, Shares, Outstanding at Sep. 30, 2015	71,735,129		71,735,129
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Sep. 30, 2015	$ 1,758,686	$ 140	$ 717	$ 1,866,605	$ (11,064)	$ (99,883)	$ 1,756,515	$ 2,171