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, 2016	$ 1,609,116	$ 100	$ 719	$ 1,776,404	$ (2,312)	$ (169,227)	$ 1,605,684	$ 3,432
Preferred stock, shares outstanding at Dec. 31, 2016		10,000,000
Common stock, shares, outstanding at Dec. 31, 2016			71,922,904
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares, net of offering 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 (in shares)			208,238
Share-based compensation, value	3,717		$ 1	2,889			2,890	827
Distributions on common shares/units	(79,537)					(79,268)	(79,268)	(269)
Distributions on preferred shares	(12,085)					(12,070)	(12,070)	(15)
Net contribution from non-controlling interests	106							106
Unrealized gain (loss) on derivative instruments	1,544				1,544		1,544
Net income (loss)	88,330					87,989	87,989	341
Preferred stock, shares outstanding at Sep. 30, 2017		10,000,000
Common stock, shares, outstanding at Sep. 30, 2017			68,812,575
Ending balance at Sep. 30, 2017	1,515,651	$ 100	$ 688	1,683,785	(768)	(172,576)	1,511,229	4,422
Beginning balance at Dec. 31, 2017	$ 1,503,526	$ 100	$ 688	1,685,437	3,689	(191,013)	1,498,901	4,625
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares, net of offering costs	$ (470)			(470)			(470)
Issuance of common shares for Board of Trustees compensation, Shares			17,410
Issuance of common shares for Board of Trustees compensation	662		$ 1	661			662
Repurchase of common shares, Shares			(69,687)
Repurchase of common shares	(2,507)		$ (1)	(2,506)			(2,507)
Share-based compensation (in shares)			151,887
Share-based compensation, value	4,237		$ 1	3,408			3,409	828
Distributions on common shares/units	(79,226)					(78,957)	(78,957)	(269)
Distributions on preferred shares	(12,086)					(12,070)	(12,070)	(16)
Unrealized gain (loss) on derivative instruments	8,119				8,119		8,119
Net income (loss)	$ 112,728					112,304	112,304	424
Preferred stock, shares outstanding at Sep. 30, 2018	10,000,000	10,000,000
Common stock, shares, outstanding at Sep. 30, 2018	68,912,185		68,912,185
Ending balance at Sep. 30, 2018	$ 1,534,983	$ 100	$ 689	$ 1,686,530	12,356	(170,284)	$ 1,529,391	$ 5,592
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect adjustment from adoption of new accounting standard					$ 548	$ (548)