Stockholders' Equity - Capitalization and IPO (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 08, 2017	Oct. 04, 2016	Sep. 30, 2018	Dec. 31, 2017
Capitalization and Initial Public Offering
Common stock authorized shares		100,000,000	100,000,000	100,000,000
Shares of common stock, par value (in dollars per share)		$ 0.0001	$ 0.0001	$ 0.0001
Shares of undesignated preferred stock, authorized		10,000,000	10,000,000	10,000,000
Shares of undesignated preferred stock, par value (in dollars per share)		$ 0.0001	$ 0.0001	$ 0.0001
Proceeds from issuance of common stock, net of underwriting costs	$ 34,897
Underwriting discounts and commission	$ 2,228
IPO
Capitalization and Initial Public Offering
Issuance of common stock upon public offering, net of issuance costs (in shares)		4,300,000
Issuance price (in dollars per share)		$ 12.00
Proceeds from issuance of common stock under initial public offering, net of underwriting costs		$ 55,186
Underwriting discounts and commissions		$ 4,154
Underwriter option
Capitalization and Initial Public Offering
Issuance of common stock upon public offering, net of issuance costs (in shares)		645,000
Common Stock
Capitalization and Initial Public Offering
Redesignation of Class A and Class B common stock upon initial public offering (in shares)		5,583,405
Conversion of redeemable convertible preferred stock upon initial public offering (in shares)		5,089,436
Underwritten public offering (n shares)	1,350,000
Share price (in dollars per share)	$ 27.50