Organization - Additional Information (Details) $ / shares in Units, $ in Thousands				3 Months Ended	9 Months Ended
	Jul. 10, 2020 USD ($) $ / shares shares	Oct. 10, 2019 USD ($) $ / shares shares	Sep. 16, 2019	Sep. 30, 2020 USD ($) shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($) shares	Dec. 31, 2019 USD ($)	Oct. 11, 2019 shares	Jun. 30, 2019 shares	Dec. 31, 2018 shares	Sep. 30, 2016 shares
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Reverse stock split, conversion ratio			0.222222222
Reverse stock split, description					On September 16, 2019, the Company’s board of directors approved an amendment to the Company’s amended and restated certificate of incorporation to effect a 1-for-4.5 reverse split (“Reverse Split”) of shares of the Company’s common and convertible preferred stock, which was effected on September 27, 2019.
Net proceeds after deducting underwriting discounts, commissions and offering expenses		$ 126,400
Gross proceeds from the offering				$ 323,214	$ 323,214
Offering expenses						$ 3,686
Accumulated deficit				561,535	561,535		$ 368,519
Cash, cash equivalents and short-term investments				$ 826,600	$ 826,600
Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Outstanding convertible preferred stock (in shares) | shares						88,112,733			88,112,733	69,910,520
Series A-1 Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Warrant to purchase of convertible preferred stock | shares											244,444
Common Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Issuance of common stock, shares | shares				8,214,285	8,214,285
Warrant to purchase of convertible preferred stock | shares		244,444
Gross proceeds from the offering				$ 1	$ 1
IPO
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Issuance of common stock, shares | shares		7,142,858
Common stock price per share | $ / shares		$ 20.00
Outstanding convertible preferred stock (in shares) | shares		88,112,733						0
IPO | Common Stock | Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of convertible preferred stock converted to shares of common stock (in shares) | shares		88,112,733
IPO | Common Stock | Warrant to Purchase Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of convertible preferred stock converted to shares of common stock (in shares) | shares		244,444
Follow-on Offering | Common Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Issuance of common stock, shares | shares	8,214,285
Common stock price per share | $ / shares	$ 42.00
Gross proceeds from the offering	$ 345,000
Payments of underwriting discounts and commissions	20,700
Offering expenses	1,100
Net proceeds after deducting underwriting discounts, commissions and offering expenses	$ 323,200