Stock-Based Compensation Expense - Schedule of Fair Value of Stock Option Estimated on Date of Grant Using Black-Scholes Option-Pricing Model (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Risk-free interest rate	0.90%	0.30%	0.70%	0.60%
Expected term (in years)	6 years 1 month 6 days	6 years 1 month 6 days	6 years	6 years
Expected volatility	84.20%	82.80%	85.00%	80.40%