Nature of the Business and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands					1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 17, 2018 USD ($) $ / shares shares	Nov. 14, 2017 USD ($) $ / shares shares	Nov. 06, 2017 USD ($) $ / shares shares	Oct. 20, 2017	Nov. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) shares	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) shares	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Nature Of Business And Basis Of Presentation [Line Items]
Entity incorporation month and year								2013-12
State of incorporation								Delaware
Merger date								Jun. 30, 2017
Business combination exchange ratio								1
Net loss						$ 10,463	$ 9,836	$ 31,063	$ 24,881	$ 38,700	$ 25,500
Accumulated deficit						$ 127,903		$ 127,903		$ 96,840
Underwritten Public Offering [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Net proceeds from issuance of common stock	$ 70,500
Offering expenses payable	$ 1,000
Underwritten Public Offering [Member] | Series A Convertible Preferred Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Issuance of stock, shares | shares	2,220
Share price | $ / shares	$ 12,500
Convertible preferred stock, description	Each share of Series A Convertible Preferred Stock sold in the offering is convertible into 1,000 shares of the Company’s common stock.
Number of shares issuable upon conversion | shares	1,000					1,000		1,000
Common Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Reverse stock split ratio				0.164544
Reverse stock split				One-for-6.0774
Net proceeds from issuance of common stock		$ 6,100	$ 71,600		$ 74,200
Common Stock [Member] | Initial Public Offering [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Issuance of stock, shares | shares		471,498	5,500,000
Share price | $ / shares		$ 14.00	$ 14.00
Common Stock [Member] | Underwritten Public Offering [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Issuance of stock, shares | shares	3,780,000
Share price | $ / shares	$ 12.50