Consolidated Statements of Equity (USD $) In Thousands, except Share data, unless otherwise specified	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]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Dec. 31, 2012	$ 1,313,092	$ 90	$ 610	$ 1,362,349	$ (300)	$ (49,798)	$ 1,312,951	$ 141
Preferred Stock, Shares Outstanding at Dec. 31, 2012		9,000,000
Common Stock, Shares, Outstanding at Dec. 31, 2012			60,955,090
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares, net of offering costs, Shares		4,000,000	171,293
Issuance of shares, net of offering costs, Value	101,234	40	2	101,192			101,234
Issuance of Common Shares for Board Of Trustees Compensation , Shares			9,097
Issuance of Common Shares for Board Of Trustees Compensation, Value	207			207			207
Repurchase of Common Stock, Shares			(21,644)
Repurchase of Common Shares, Value	(523)			(523)			(523)
Share-based Compensation, Shares			65,192
Share-based Compensation, Value	2,469			1,679			1,679	790
Distributions on Common Shares and Units	(19,786)					(19,664)	(19,664)	(122)
Distributions on Preferred Shares	(10,781)					(10,772)	(10,772)	(9)
Unrealized gain (loss) on derivative instruments	1,800				1,800		1,800
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	14,685					14,586	14,586	99
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Jun. 30, 2013	1,402,397	130	612	1,464,904	1,500	(65,648)	1,401,498	899
Preferred Stock, Shares Outstanding at Jun. 30, 2013		13,000,000
Common Stock, Shares, Outstanding at Jun. 30, 2013			61,179,028
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
Preferred Stock, Shares Outstanding at Dec. 31, 2013	13,000,000	13,000,000
Common Stock, Shares, Outstanding at Dec. 31, 2013	63,709,628		63,709,628
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	(188)	0	0	(188)			(188)
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			1
Share-based Compensation, Value	4,766			3,424			3,425	1,341
Distributions on Common Shares and Units	(30,030)					(29,750)	(29,750)	(280)
Distributions on Preferred Shares	(12,171)					(12,163)	(12,163)	(8)
Unrealized gain (loss) on derivative instruments	(578)				(578)		(578)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	27,014					26,751	26,751	263
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Jun. 30, 2014	$ 1,463,686	$ 130	$ 638	$ 1,544,163	$ 508	$ (84,814)	$ 1,460,625	$ 3,061
Preferred Stock, Shares Outstanding at Jun. 30, 2014	13,000,000	13,000,000
Common Stock, Shares, Outstanding at Jun. 30, 2014	63,764,929		63,764,929