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Description of the Business - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands |  |  |  | 36 Months Ended |
--- | --- | --- | --- | --- | ---
May 21, 2020 | Feb. 12, 2019 | Jan. 04, 2018 | Sep. 30, 2020 | Dec. 31, 2019
Description Of Business [Line Items] |  |  |  |  |
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs (in shares) | 9,433,963 |  |  |  |
Offering price (in dollars per share) | $ 13.25 |  |  |  |
Accumulated deficit |  |  |  | $ (512,926) | $ (334,170)
Funds raised through Series A and Series B Convertible Preferred Stock, convertible note financings and completed IPO |  |  |  | $ 942,000 |
AA Biopharma Inc. |  |  |  |  |
Description Of Business [Line Items] |  |  |  |  |
Cash received in connection with acquisition |  |  | $ 12,800 |  |
2027 Notes |  |  |  |  |
Description Of Business [Line Items] |  |  |  |  |
Debt instrument interest rate percentage | 5.00% |  |  | 5.00% |
Initial Public Offering ("IPO") |  |  |  |  |
Description Of Business [Line Items] |  |  |  |  |
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs (in shares) |  | 19,837,500 |  |  |
Offering price (in dollars per share) |  | $ 16.00 |  |  |
Net proceeds, after deducting underwriting discounts, commissions, and offering expenses |  | $ 291,300 |  |  |
Initial Public Offering ("IPO") | Conversion of Convertible Preferred Stock into Common Stock |  |  |  |  |
Description Of Business [Line Items] |  |  |  |  |
Common stock issued in conversion of convertible preferred stock (in shares) |  | 30,493,460 |  |  |

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