Consolidated Statements of Equity - USD ($) $ in Thousands	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]	Non-controlling Interests [Member]
Beginning balance at Dec. 31, 2014	$ 1,782,411	$ 140	$ 716	$ 1,864,739	$ (341)	$ (84,163)	$ 1,781,091	$ 1,320
Preferred stock, shares outstanding at Dec. 31, 2014		14,000,000
Common stock, shares, outstanding at Dec. 31, 2014			71,553,481
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares, net of offering costs, Shares		0	0
Issuance of shares, net of offering costs		$ 0	$ 0
Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs	(195)			(195)			(195)
Issuance of common shares for Board of Trustees compensation, Shares			8,084
Issuance of common shares for Board of Trustees compensation	372			372			372
Repurchase of common shares, Shares			(84,835)
Repurchase of common shares	(4,094)			(4,094)			(4,094)
Share-based compensation, Shares			258,399
Share-based compensation	8,331		$ 1	7,225			7,226	1,105
Distributions on common shares/units	(90,612)					(90,320)	(90,320)	(292)
Distributions on preferred shares	(25,965)					(25,950)	(25,950)	(15)
Unrealized gain (loss) on derivative instruments	(4,409)				(4,409)		(4,409)
Net income (Loss)	94,995					94,668	94,668	327
Preferred stock, shares outstanding at Dec. 31, 2015		14,000,000
Common stock, shares, outstanding at Dec. 31, 2015			71,735,129
Ending balance at Dec. 31, 2015	1,760,834	$ 140	$ 717	1,868,047	(4,750)	(105,765)	1,758,389	2,445
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares, net of offering costs, Shares		5,000,000	0
Issuance of shares, net of offering costs	120,808	$ 50	$ 0	120,758			120,808
Redemption of preferred shares, Shares		(9,000,000)
Redemption of preferred shares	(225,050)	$ (90)		(217,870)		(7,090)	(225,050)
Issuance of common shares for Board of Trustees compensation, Shares			21,407
Issuance of common shares for Board of Trustees compensation	606			606			606
Repurchase of common shares, Shares			(88,510)
Repurchase of common shares	(2,496)		$ (1)	(2,495)			(2,496)
Share-based compensation, Shares			254,878
Share-based compensation	8,466		$ 3	7,358			7,361	1,105
Distributions on common shares/units	(110,773)					(110,414)	(110,414)	(359)
Distributions on preferred shares	(19,679)					(19,662)	(19,662)	(17)
Unrealized gain (loss) on derivative instruments	2,438				2,438		2,438
Net income (Loss)	$ 73,962					73,704	73,704	258
Preferred stock, shares outstanding at Dec. 31, 2016	10,000,000	10,000,000
Common stock, shares, outstanding at Dec. 31, 2016	71,922,904		71,922,904
Ending balance at Dec. 31, 2016	$ 1,609,116	$ 100	$ 719	1,776,404	(2,312)	(169,227)	1,605,684	3,432
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares, net of offering costs, Shares		0
Issuance of shares, net of offering costs		$ 0
Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs	(62)			(62)			(62)
Issuance of common shares for Board of Trustees compensation, Shares			16,711
Issuance of common shares for Board of Trustees compensation	503		$ 1	502			503
Repurchase of common shares, Shares			(3,335,278)
Repurchase of common shares	(95,981)		$ (33)	(95,948)			(95,981)
Share-based compensation, Shares			208,238
Share-based compensation	5,646		$ 1	4,541			4,542	1,104
Distributions on common shares/units	(105,939)					(105,580)	(105,580)	(359)
Distributions on preferred shares	(16,126)					(16,094)	(16,094)	(32)
Net contribution from non-controlling interests	106							106
Unrealized gain (loss) on derivative instruments	6,001				6,001		6,001
Net income (Loss)	$ 100,262					99,888	99,888	374
Preferred stock, shares outstanding at Dec. 31, 2017	10,000,000	10,000,000
Common stock, shares, outstanding at Dec. 31, 2017	68,812,575		68,812,575
Ending balance at Dec. 31, 2017	$ 1,503,526	$ 100	$ 688	$ 1,685,437	$ 3,689	$ (191,013)	$ 1,498,901	$ 4,625