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Description of the Business - Additional Information (Detail) |  |  |  | 18 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
May 02, 2019 USD ($) Tranche | Feb. 12, 2019 USD ($) $ / shares shares | Jan. 04, 2018 USD ($) | Mar. 31, 2019 USD ($) | Sep. 30, 2019 USD ($) Tranche | Dec. 31, 2018 USD ($)
Description Of Business [Line Items] |  |  |  |  |  |
Accumulated deficit |  |  |  |  | $ (279,472,000) | $ (153,863,000)
Funds raised through Series A and Series B Convertible Preferred Stock, convertible note financings and completed IPO |  |  |  | $ 601,300,000 |  |
Term Loan |  |  |  |  |  |
Description Of Business [Line Items] |  |  |  |  |  |
Maximum borrowing capacity | $ 150,000,000 |  |  |  |  |
Amount funded | $ 30,000,000 |  |  |  |  |
Number of additional tranches executed | Tranche | 3 |  |  |  | 0 |
Remaining borrowing capacity | $ 120,000,000 |  |  |  |  |
Term Loan | Tranche One |  |  |  |  |  |
Description Of Business [Line Items] |  |  |  |  |  |
Remaining borrowing capacity | 40,000,000 |  |  |  |  |
Term Loan | Tranche Two |  |  |  |  |  |
Description Of Business [Line Items] |  |  |  |  |  |
Remaining borrowing capacity | 30,000,000 |  |  |  |  |
Term Loan | Tranche Three |  |  |  |  |  |
Description Of Business [Line Items] |  |  |  |  |  |
Remaining borrowing capacity | $ 50,000,000 |  |  |  |  |
AA Biopharma Inc. |  |  |  |  |  |
Description Of Business [Line Items] |  |  |  |  |  |
Cash received in connection with acquisition |  |  | $ 12,800,000 |  |  |
Initial Public Offering ("IPO") |  |  |  |  |  |
Description Of Business [Line Items] |  |  |  |  |  |
Sale of shares of common stock | shares |  | 19,837,500 |  |  |  |
Offering price, per share | $ / shares |  | $ 16.00 |  |  |  |
Net proceeds, after deducting underwriting discounts, commissions, and offering expenses |  | $ 291,300,000 |  |  |  |
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 | shares |  | 30,493,460 |  |  |  |

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