Common Stock - Reserved for Future Issuance (Details) - shares	Dec. 31, 2018	Aug. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Common stock shares reserved for future issuance (in shares)	37,219,376		89,292,466
2014 Equity Incentive Plan
Class of Stock [Line Items]
Common stock shares reserved for future issuance (in shares)	0		3,962,024
2018 Equity Incentive Plan
Class of Stock [Line Items]
Common stock shares reserved for future issuance (in shares)	10,558,306	10,701,505	0
2018 Employee Stock Purchase Plan
Class of Stock [Line Items]
Common stock shares reserved for future issuance (in shares)	1,700,000	1,700,000	0
Warrant to purchase redeemable convertible preferred stock
Class of Stock [Line Items]
Common stock shares reserved for future issuance (in shares)	0		398,331
Warrant to purchase common stock
Class of Stock [Line Items]
Common stock shares reserved for future issuance (in shares)	898,331		45,286
Conversion of redeemable convertible preferred stock
Class of Stock [Line Items]
Common stock shares reserved for future issuance (in shares)	0		61,279,079
Options issued and outstanding
Class of Stock [Line Items]
Common stock shares reserved for future issuance (in shares)	23,774,279		23,607,746
RSUs issued and outstanding
Class of Stock [Line Items]
Common stock shares reserved for future issuance (in shares)	288,460		0