Stock Incentive Plans - Summary of Unvested Common Stock from Early Exercises Subject to Repurchase (Details)	9 Months Ended
	Sep. 30, 2018 shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Unvested shares - January 1, 2018	265,098
Vested	(97,250)
Exercised	16,150
Unvested shares - September 30, 2018	183,998