Stock Option Plan - Summary of Unvested Common Stock from Early Exercises Subject to Repurchase (Details)	12 Months Ended
	Dec. 31, 2018 shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Unvested shares—Beginning Balance	265,098
Vested	(129,959)
Issued	16,150
Unvested shares—Ending Balance	151,289