Consolidated Statements of Equity (USD $)	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, 2011	$ 1,116,348,000	$ 90,000	$ 508,000	$ 1,142,905,000	$ 0	$ (30,252,000)	$ 1,113,251,000	$ 3,097,000
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			9,694,087
Issuance of shares, net of offering costs, Value	214,871,000		97,000	214,774,000			214,871,000
Issuance of Common Shares for Board Of Trustees Compensation , Shares			10,361
Issuance of Common Shares for Board Of Trustees Compensation, Value	199,000			199,000			199,000
Repurchase of Common Stock, Shares			(15,706)
Repurchase of Common Shares, Value	(321,000)			(321,000)			(321,000)
Share-based Compensation, Shares			52,789
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures			1,000
Share-based Compensation, Value	4,215,000			2,635,000			2,636,000	1,579,000
Distributions on Common Shares and Units	(28,246,000)					(27,800,000)	(27,800,000)	(446,000)
Distributions on Preferred Shares	(17,840,000)					(17,825,000)	(17,825,000)	(15,000)
Unrealized gain (loss) on derivative instruments	(300,000)				(300,000)		(300,000)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	26,508,000					26,079,000	26,079,000	429,000
Stock Issued During Period, Shares, Conversion of Units			444,535
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests	(2,342,000)		4,000	2,157,000			2,161,000	(4,503,000)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Dec. 31, 2012	1,313,092,000	90,000	610,000	1,362,349,000	(300,000)	(49,798,000)	1,312,951,000	141,000
Common Stock, Shares, Outstanding at Dec. 31, 2012			60,955,090
Preferred Stock, Shares Outstanding at Dec. 31, 2012		9,000,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares, net of offering costs, Shares		4,000,000	2,701,893
Issuance of shares, net of offering costs, Value	175,575,000	40,000	27,000	175,508,000			175,575,000
Issuance of Common Shares for Board Of Trustees Compensation , Shares			9,097
Issuance of Common Shares for Board Of Trustees Compensation, Value	207,000			207,000			207,000
Repurchase of Common Stock, Shares			(21,644)
Repurchase of Common Shares, Value	(523,000)			(523,000)			(523,000)
Share-based Compensation, Shares			65,192
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures			0
Share-based Compensation, Value	5,222,000			3,597,000			3,597,000	1,625,000
Distributions on Common Shares and Units	(40,099,000)					(39,819,000)	(39,819,000)	(280,000)
Distributions on Preferred Shares	(22,968,000)					(22,953,000)	(22,953,000)	(15,000)
Unrealized gain (loss) on derivative instruments	1,386,000				1,386,000		1,386,000
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	43,192,000					42,918,000	42,918,000	274,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Dec. 31, 2013	1,475,084,000	130,000	637,000	1,541,138,000	1,086,000	(69,652,000)	1,473,339,000	1,745,000
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		1,000,000	7,530,000
Issuance of shares, net of offering costs, Value	316,910,000	10,000	75,000	316,825,000			316,910,000
Issuance of Common Shares for Board Of Trustees Compensation , Shares			13,793
Issuance of Common Shares for Board Of Trustees Compensation, Value	421,000			421,000			421,000
Repurchase of Common Stock, Shares			(20,539)
Repurchase of Common Shares, Value	(632,000)			(632,000)			(632,000)
Share-based Compensation, Shares			62,498
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures			1,000
Share-based Compensation, Value	11,695,000			9,085,000			9,086,000	2,609,000
Distributions on Common Shares and Units	(62,857,000)					(62,298,000)	(62,298,000)	(559,000)
Distributions on Preferred Shares	(25,095,000)					(25,079,000)	(25,079,000)	(16,000)
Unrealized gain (loss) on derivative instruments	(1,427,000)				(1,427,000)		(1,427,000)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	73,543,000					72,866,000	72,866,000	677,000
Stock Issued During Period, Shares, Conversion of Units			258,101
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests	(5,231,000)		3,000	(2,098,000)			(2,095,000)	(3,136,000)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Dec. 31, 2014	$ 1,782,411,000	$ 140,000	$ 716,000	$ 1,864,739,000	$ (341,000)	$ (84,163,000)	$ 1,781,091,000	$ 1,320,000
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