Formation and Business of the Company (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Jan. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Schedule of Capitalization, Equity [Line Items]
Net loss		$ (12,847)	$ (8,172)	$ (33,852)	$ (19,800)
Net cash used in operating activities				(36,116)	$ (18,065)
Cash, cash equivalents and investments		152,700		152,700
Accumulated deficit		$ (93,042)		$ (93,042)		$ (59,191)
IPO
Schedule of Capitalization, Equity [Line Items]
Shares issued (shares)	8,050,000
Proceeds from issuance initial public offering, net	$ 125,400
Over-Allotment Option
Schedule of Capitalization, Equity [Line Items]
Shares issued (shares)	1,050,000
Convertible Stock Converted into Shares of Common Stock
Schedule of Capitalization, Equity [Line Items]
Common shares issued upon conversion of convertible stock (shares)	9,629,405