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			1,407,079
Issuance of shares, net of offering costs, Value	31,873		14	31,859			31,873
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	318			(318)			(318)
Share-based Compensation, Shares			48,324
Share-based Compensation, Value	853			457			457	396
Distributions on Common Shares and Units	(6,395)					(6,282)	(6,282)	(113)
Distributions on Preferred Shares	(4,456)					(4,456)	(4,456)
Unrealized gain (loss) on derivative instruments	0
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	(2,837)					(2,791)	(2,791)	(46)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Mar. 31, 2012	1,135,267	90	522	1,175,102		(43,781)	1,131,933	3,334
Preferred Stock, Shares Outstanding at Mar. 31, 2012		9,000,000
Common Stock, Shares, Outstanding at Mar. 31, 2012			52,219,193
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		3,600,000
Issuance of shares, net of offering costs, Value	86,965	36		86,929			86,965
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	1,230			835			835	395
Distributions on Common Shares and Units	(9,886)					(9,825)	(9,825)	(61)
Distributions on Preferred Shares	(4,668)					(4,668)	(4,668)	0
Unrealized gain (loss) on derivative instruments	105				105		105
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	(246)					(248)	(248)	2
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Mar. 31, 2013	$ 1,386,276	$ 126	$ 610	$ 1,449,797	$ (195)	$ (64,539)	$ 1,385,799	$ 477
Preferred Stock, Shares Outstanding at Mar. 31, 2013	12,600,000	12,600,000
Common Stock, Shares, Outstanding at Mar. 31, 2013	61,007,735		61,007,735