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			(2,210,690)
Repurchase of common shares	(62,444)		$ (22)	(62,422)			(62,444)
Share-based compensation (in shares)			208,238
Share-based compensation, value			$ 1
Share-based compensation	1,127			850			851	276
Distributions on common shares/units	(26,531)					(26,441)	(26,441)	(90)
Distributions on preferred shares	(4,023)					(4,023)	(4,023)
Net contribution from non-controlling interests	107							107
Unrealized gain (loss) on derivative instruments	2,104				2,104		2,104
Net income (loss)	14,089					14,034	14,034	55
Preferred stock, shares outstanding at Mar. 31, 2017		10,000,000
Common stock, shares, outstanding at Mar. 31, 2017			69,937,163
Ending balance at Mar. 31, 2017	1,533,986	$ 100	$ 699	1,715,272	(208)	(185,657)	1,530,206	3,780
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 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			$ 1
Share-based compensation	(269)			(546)			(545)	276
Distributions on common shares/units	(26,274)					(26,184)	(26,184)	(90)
Distributions on preferred shares	(4,023)					(4,023)	(4,023)
Unrealized gain (loss) on derivative instruments	5,278				5,278		5,278
Unrealized gain (loss) on marketable securities	(579)				(579)		(579)
Net income (loss)	$ 24,516					24,409	24,409	107
Preferred stock, shares outstanding at Mar. 31, 2018	10,000,000	10,000,000
Common stock, shares, outstanding at Mar. 31, 2018	68,912,185		68,912,185
Ending balance at Mar. 31, 2018	$ 1,500,330	$ 100	$ 689	$ 1,683,046	8,936	(197,359)	$ 1,495,412	$ 4,918
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect adjustment from adoption of new accounting standard					$ 548	$ (548)