Consolidated Statements of Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Cumulative Distributions	Accumulated Deficit	Accumulated Other Comprehensive Income	Total Stockholders’ Equity	Non- controlling Interests
Beginning balance at Dec. 31, 2016	$ 554,455	$ 71	$ 615,653	$ (38,406)	$ (23,788)	$ 841	$ 554,371	$ 84
Beginning balance (shares) at Dec. 31, 2016		70,939,647
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Gross proceeds from issuance of common stock	41,826	$ 4	41,822				41,826
Gross proceeds from issuance of common stock (shares)		4,205,673
Discount on issuance of common stock	(16)		(16)				(16)
Stock-based compensation	292		292				292
Stock-based compensation (in shares)		25,500
Offering costs including dealer manager fees and stockholder servicing fees to affiliates	(3,593)		(3,593)				(3,593)
Distributions to common stockholders	(19,427)			(19,427)			(19,427)
Issuance of shares for distribution reinvestment plan		$ 2	22,206	(22,208)
Issuance of shares for distribution reinvestment plan (shares)		2,358,188
Repurchase of common stock	(5,742)	$ (1)	(5,741)				(5,742)
Repurchase of common stock (in shares)		(623,499)
Repurchase of common stock	(16,467)		(16,467)				(16,467)
Issuance of stock dividends	0	$ 0	2,549	(2,549)
Issuance of stock dividends (in shares)		269,774
Operating partnership units issued								0
Distributions to noncontrolling interest	(11)							(11)
Net income	11,119				11,116		11,116	3
Other comprehensive loss	108					108	108
Ending balance at Dec. 31, 2017	$ 562,544	$ 76	656,705	(82,590)	(12,672)	949	562,468	76
Ending balance (shares) at Dec. 31, 2017	77,175,283	77,175,283
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Gross proceeds from issuance of common stock	$ 2,196	$ 0	2,196				2,196
Gross proceeds from issuance of common stock (shares)		225,388
Discount on issuance of common stock	(28)		(28)				(28)
Stock-based compensation	44		44				44
Stock-based compensation (in shares)		10,500
Offering costs including dealer manager fees and stockholder servicing fees to affiliates	(445)		(445)				(445)
Distributions to common stockholders	(10,135)			(10,135)			(10,135)
Issuance of shares for distribution reinvestment plan	10,970	$ 1	10,969	(10,970)
Issuance of shares for distribution reinvestment plan (shares)		1,145,544
Repurchase of common stock	(6,581)	$ (1)	(6,580)				(6,581)
Repurchase of common stock (in shares)		(721,309)
Repurchase of common stock	(4,390)		(4,390)				(4,390)
Operating partnership units issued	1,185							1,185
Distributions to noncontrolling interest	(30)							(30)
Net income	326				326		326	0
Other comprehensive loss	(293)					(292)	(292)
Ending balance at Jun. 30, 2018	$ 544,393	$ 76	$ 658,471	$ (103,695)	$ (12,346)	$ 657	$ 543,163	$ 1,230
Ending balance (shares) at Jun. 30, 2018	77,835,406	77,835,406