Stock-Based Incentive Compensation Plans - Narrative (Details) $ in Thousands	3 Months Ended				6 Months Ended	9 Months Ended
	Sep. 30, 2020 USD ($) shares	Jun. 30, 2020 USD ($)	Mar. 25, 2020	Jun. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) purchase_period shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($) shares	Dec. 31, 2019 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost, stock options	$ 3,700				$ 3,700	$ 3,700
Stock options granted (in shares) | shares	0					26,236
Early exercise of stock options, share subject to repurchase (in shares) | shares	7,784				7,784	7,784		21,404
Issuance of common stock related to employee stock purchase plan (in shares) | shares						74,685	99,086
Issuance of common stock related to employee stock purchase plan		$ 991		$ 1,263		$ 1,000	$ 1,300
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost, expected period for recognition (in years)						2 years 1 month 6 days
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost, expected period for recognition (in years)						3 years
Unrecognized compensation cost, RSUs	$ 20,900				$ 20,900	$ 20,900
2018 ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Purchase price of common stock as a percent of fair market value (percent)						85.00%
Employee stock purchase program offering period interval (in months)			6 months
Accrued compensation and related expenses for employee payroll deductions	$ 700				$ 700	$ 700		$ 200
2018 Amended ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee stock purchase program offering period interval (in months)					12 months
Number of purchase periods in offering interval | purchase_period					2