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			9,544,087
Issuance of shares, net of offering costs, Value	211,520		95	211,425			211,520
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
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures			1
Share-based Compensation, Value	2,750			1,564			1,565	1,185
Distributions on Common Shares and Units	(20,810)					(20,476)	(20,476)	(334)
Distributions on Preferred Shares	(13,378)					(13,369)	(13,369)	(9)
Unrealized gain (loss) on derivative instruments	(389)				(389)		(389)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	19,295					18,991	18,991	304
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Sep. 30, 2012	1,315,216	90	604	1,355,774		(45,106)	1,310,973	4,243
Preferred Stock, Shares Outstanding at Sep. 30, 2012		9,000,000
Common Stock, Shares, Outstanding at Sep. 30, 2012			60,356,201
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	171,893
Issuance of shares, net of offering costs, Value	101,242	40	2	101,200			101,242
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	3,857			2,672			2,672	1,185
Distributions on Common Shares and Units	(29,705)					(29,522)	(29,522)	(183)
Distributions on Preferred Shares	(16,881)					(16,872)	(16,872)	(9)
Unrealized gain (loss) on derivative instruments	1,211				1,211		1,211
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	32,212					32,001	32,001	211
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Sep. 30, 2013	$ 1,404,712	$ 130	$ 612	$ 1,465,905	$ 911	$ (64,191)	$ 1,403,367	$ 1,345
Preferred Stock, Shares Outstanding at Sep. 30, 2013	13,000,000	13,000,000
Common Stock, Shares, Outstanding at Sep. 30, 2013	61,179,628		61,179,628