Redeemable Convertible Preferred Stock and Stockholders' Equity - Summary of Common Stock Reserved for Future Issuance (Details) - shares	Sep. 30, 2018	Dec. 31, 2017	Jul. 26, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options issued and outstanding (in shares)	10,150,522	13,180,950
Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options issued and outstanding (in shares)	10,150,522	13,180,950
Restricted stock units outstanding (in shares)	2,081,173	981,276
Shares available for future equity grants (in shares)	6,214,739	7,026,071
Total common stock reserved (in shares)	18,446,434	21,188,297	7,898,159
2017 Employee Stock Purchase Plan:
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued under employee stock purchase plan (in shares)	187,076	0
Shares available for future purchases under employee stock purchase plan (in shares)	2,227,612	1,600,000
Total common stock reserved (in shares)	2,414,688	1,600,000