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]	Retained Earnings [Member]	Parent [Member]	Noncontrolling Interest [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Dec. 31, 2011	$ 1,116,348	$ 90	$ 508	$ 1,142,905		$ (30,252)	$ 1,113,251	$ 3,097
Preferred Stock, Shares Outstanding at Dec. 31, 2011		9,000,000
Common Stock, Shares, Outstanding at Dec. 31, 2011			50,769,024
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares, net of offering costs, Shares			6,619,527
Issuance of shares, net of offering costs, Value	142,292		66	142,226			142,292
Issuance of Common Shares for Board Of Trustees Compensation , Shares			10,361
Issuance of Common Shares for Board Of Trustees Compensation, Value	199			199			199
Repurchase of Common Stock, Shares			(15,595)
Repurchase of Common Shares, Value	(319)			(319)			(319)
Share-based Compensation, Shares			48,324
Share-based Compensation, Value	1,801			1,011			1,011	790
Distributions on Common Shares and Units	(13,414)					(13,191)	(13,191)	(223)
Distributions on Preferred Shares	(8,921)					(8,913)	(8,913)	(8)
Unrealized gain (loss) on derivative instruments	0
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	7,190					7,073	7,073	117
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Jun. 30, 2012	1,245,176	90	574	1,286,022		(45,283)	1,241,403	3,773
Preferred Stock, Shares Outstanding at Jun. 30, 2012		9,000,000
Common Stock, Shares, Outstanding at Jun. 30, 2012			57,431,641
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	9,000,000
Common Stock, Shares, Outstanding at Dec. 31, 2012	60,955,090		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	13,000,000
Common Stock, Shares, Outstanding at Jun. 30, 2013	61,179,028		61,179,028