CONSOLIDATED STATEMENTS OF EQUITY - USD ($) $ in Thousands	Total	Total Stockholders’ Equity	Common Stock	Additional Paid-in Capital	Cumulative Distributions and Net Income (Losses)	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interest
Balance (in shares) at Dec. 31, 2016			180,890,572
Balance at Dec. 31, 2016	$ 1,193,376	$ 1,193,076	$ 1,809	$ 1,591,652	$ (398,087)	$ (2,298)	$ 300
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,374	1,374			1,374
Other comprehensive income (loss)	2,408	2,408				2,408
Issuance of common stock (in shares)			5,919,223
Issuance of common stock	59,785	59,785	$ 59	59,726
Transfers from redeemable common stock	2,086	2,086		2,086
Redemptions of common stock (in shares)			(5,945,088)
Redemptions of common stock	(61,871)	(61,871)	$ (59)	(61,812)
Distributions declared	(117,738)	(117,738)			(117,738)
Other offering costs	$ (12)	(12)		(12)
Balance (in shares) at Dec. 31, 2017	180,864,707		180,864,707
Balance at Dec. 31, 2017	$ 1,079,408	1,079,108	$ 1,809	1,591,640	(514,451)	110	300
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	30,309	30,309			30,309		0
Other comprehensive income (loss)	(110)	(110)				(110)
Issuance of common stock (in shares)			3,806,525
Issuance of common stock	42,443	42,443	$ 38	42,405
Transfers from redeemable common stock	8,813	8,813		8,813
Redemptions of common stock (in shares)			(8,248,444)
Redemptions of common stock	(94,569)	(94,569)	$ (83)	(94,486)
Distributions declared	(86,394)	(86,394)			(86,394)
Other offering costs	$ (2)	(2)		(2)
Balance (in shares) at Sep. 30, 2018	176,422,788		176,422,788
Balance at Sep. 30, 2018	$ 979,898	$ 979,598	$ 1,764	$ 1,548,370	$ (570,536)	$ 0	$ 300