Equity Incentive Plans, Employee Stock Purchase Plan and Stock-Based Compensation - Schedule of Fair Value of ESPP Shares Using Option Pricing Model (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, Minimum	73.20%	72.70%	54.60%
Expected volatility, Maximum	87.40%	74.90%	62.40%
Risk-free interest rate, Minimum	0.20%	1.40%	2.70%
Risk-free interest rate, Maximum	1.60%	2.60%	3.10%
Expected dividend	0.00%
2018 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 months	6 months	6 months
Expected volatility			55.30%
Expected volatility, Minimum	63.00%	63.00%
Expected volatility, Maximum	111.40%	83.20%
Risk-free interest rate			2.40%
Risk-free interest rate, Minimum	0.10%	1.90%
Risk-free interest rate, Maximum	1.90%	2.50%