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, 2012	$ 1,313,092,000	$ 90,000	$ 610,000	$ 1,362,349,000	$ (300,000)	$ (49,798,000)	$ 1,312,951,000	$ 141,000
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	171,893
Issuance of shares, net of offering costs, Value	101,242,000	40,000	2,000	101,200,000			101,242,000
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
Share-based Compensation, Value	3,857,000			2,672,000			2,672,000	1,185,000
Distributions on Common Shares and Units	(29,705,000)					(29,522,000)	(29,522,000)	(183,000)
Distributions on Preferred Shares	(16,881,000)					(16,872,000)	(16,872,000)	(9,000)
Unrealized gain (loss) on derivative instruments	1,211,000				1,211,000		1,211,000
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	32,212,000					32,001,000	32,001,000	211,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Sep. 30, 2013	1,404,712,000	130,000	612,000	1,465,905,000	911,000	(64,191,000)	1,403,367,000	1,345,000
Preferred Stock, Shares Outstanding at Sep. 30, 2013		13,000,000
Common Stock, Shares, Outstanding at Sep. 30, 2013			61,179,628
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	3,850,000
Issuance of shares, net of offering costs, Value	170,750,000	10,000	39,000	170,701,000			170,750,000
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			62,047
Stock Issued During Period, Value, Share-based Compensation			0
Share-based Compensation, Value	8,237,000			6,225,000			6,225,000	2,012,000
Distributions on Common Shares and Units	(46,003,000)					(45,583,000)	(45,583,000)	(420,000)
Distributions on Preferred Shares	(18,598,000)					(18,591,000)	(18,591,000)	(7,000)
Unrealized gain (loss) on derivative instruments	(132,000)				(132,000)		(132,000)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	57,453,000					56,916,000	56,916,000	537,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Sep. 30, 2014	$ 1,646,580,000	$ 140,000	$ 676,000	$ 1,717,853,000	$ 954,000	$ (76,910,000)	$ 1,642,713,000	$ 3,867,000
Preferred Stock, Shares Outstanding at Sep. 30, 2014	14,000,000	14,000,000
Common Stock, Shares, Outstanding at Sep. 30, 2014	67,614,929		67,614,929