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Stockholders' Equity - Additional Information (Detail) $ / shares in Units, $ in Thousands |  | 1 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Aug. 05, 2020 USD ($) | Apr. 30, 2020 USD ($) $ / shares | Apr. 30, 2020 USD ($) $ / shares | Apr. 30, 2019 USD ($) $ / shares shares | Jun. 30, 2018 shares | Apr. 30, 2018 USD ($) $ / shares shares | Dec. 31, 2020 USD ($) installment $ / shares shares | Dec. 31, 2019 USD ($) $ / shares shares | Dec. 31, 2018 USD ($) $ / shares shares | Jun. 30, 2020 shares | Jun. 29, 2020 shares | May 31, 2020 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Preferred shares authorized (in shares) |  |  |  |  |  |  | 5,000,000 | 5,000,000 |  |  |  |
Preferred stock issued (in shares) |  |  |  |  |  |  | 0 | 0 |  |  |  |
Preferred stock outstanding (in shares) |  |  |  |  |  |  | 0 | 0 |  |  |  |
Common stock, shares authorized (in shares) |  |  |  |  |  |  | 320,000,000 | 320,000,000 |  | 320,000,000 | 160,000,000 |
Common stock, shares outstanding (in shares) |  |  |  |  |  |  | 142,063,203 | 115,972,708 |  |  |  |
Net proceeds from issuance of common stock, after deducting offering expenses | $ |  |  |  |  |  |  | $ 0 | $ 136,308 | $ 215,758 |  |  |
Additional equity award (in shares) |  |  |  |  |  |  | 4,563,425 |  |  |  |  |
Stock option maximum term |  |  |  |  |  |  | 6 years 5 months 23 days |  |  |  |  |
Specified stockholder ownership percentage |  |  |  |  |  |  | 10.00% |  |  |  |  |
Purchase price of common stock percent under stock incentive plans for specified stockholder |  |  |  |  |  |  | 110.00% |  |  |  |  |
Stock option maximum term for specified stockholder |  |  |  |  |  |  | 5 years |  |  |  |  |
Share of common stock subject to a stock option or stock appreciation right (in shares) |  |  |  |  |  |  | 100.00% |  |  |  |  |
Intrinsic value of options exercised | $ |  |  |  |  |  |  | $ 5,400 | 4,700 | 27,000 |  |  |
Options vested and expected to vest, aggregate intrinsic value | $ |  |  |  |  |  |  | 69,600 | 7,500 | 24,500 |  |  |
Options exercisable, aggregate intrinsic value | $ |  |  |  |  |  |  | $ 29,025 |  |  |  |  |
Restricted stock units awarded (in shares) |  |  |  |  |  |  | 2,517,101 |  |  |  |  |
Total payments for the employees' tax obligations to taxing authorities | $ |  |  |  |  |  |  | $ 765 | $ 422 | $ 254 |  |  |
Shares reserved for issuance of future awards (in shares) |  |  |  |  |  |  | 2,483,218 |  |  |  |  |
Share-Based Compensation Awards, Tranche 2 |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock options granted vesting percentage |  |  |  |  |  |  | 33.33% |  |  |  |  |
Share-Based Compensation Awards, Tranche 3 |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock options granted vesting percentage |  |  |  |  |  |  | 33.33% |  |  |  |  |
2018 Stock Incentive Plan |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Additional equity award (in shares) |  |  |  |  |  | 0 |  |  |  |  |  |
Percent of fair value per share of common stock on option grant date |  |  |  |  |  |  | 100.00% |  |  |  |  |
Stock options vesting period |  |  |  |  |  |  | 4 years |  |  |  |  |
Stock options expiration term |  |  |  |  |  |  | 10 years |  |  |  |  |
Fixed ratio shares of common stock |  |  |  |  |  |  | 1.33 |  |  |  |  |
Shares reserved for issuance of future awards (in shares) |  |  |  |  |  |  | 10,942,576 |  |  |  |  |
2010 Employee Stock Purchase Plan |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Percent of fair value per share of common stock on option grant date |  |  |  |  | 85.00% |  |  |  |  |  |  |
Stock reserved for issuance under plan (in shares) |  |  |  |  | 4,600,000 |  |  |  |  |  |  |
Purchase plan offering period |  |  |  |  | 2 years |  |  |  |  |  |  |
Purchase plan purchase period |  |  |  |  | 6 months |  |  |  |  |  |  |
Restated 2018 Stock Incentive Plan |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Increase in capital shares reserved for future issuance (in shares) |  |  |  |  |  |  |  |  |  |  |  | 9,900,000
Maximum | 2018 Stock Incentive Plan |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock option maximum term |  |  |  |  |  |  | 10 years |  |  |  |  |
Employee Stock Options | 2018 Stock Incentive Plan | Share-Based Compensation Awards, Tranche 1 |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock options granted vesting percentage |  |  |  |  |  |  | 25.00% |  |  |  |  |
Employee Stock Options | 2018 Stock Incentive Plan | Share-Based Compensation Awards, Tranche 2 |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock options granted vesting percentage |  |  |  |  |  |  | 2.08% |  |  |  |  |
Restricted Stock Units (RSUs) |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Restricted stock units awarded (in shares) |  |  |  |  |  |  | 2,517,101 | 834,745 | 346,055 |  |  |
Grant date fair value per award of restricted stock units (in dollars per share) | $ / shares |  |  |  |  |  |  | $ 8.06 | $ 9.49 | $ 17.87 |  |  |
Share-based compensation arrangement by share-based payment award, number of vesting installment | installment |  |  |  |  |  |  | 3 |  |  |  |  |
Aggregate fair value of RSUs vested in period | $ |  |  |  |  |  |  | $ 3,700 | $ 2,000 | $ 600 |  |  |
Shares withheld from issuance in order to pay employee taxes (in shares) |  |  |  |  |  |  | 90,617 | 39,160 | 20,193 |  |  |
Restricted Stock Units (RSUs) | Share-Based Compensation Awards, Tranche 1 |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock options granted vesting percentage |  |  |  |  |  |  | 33.33% |  |  |  |  |
Jefferies LLC | ATM Agreement |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock, net of issuance costs (in shares) |  |  |  |  |  |  | 0 |  |  |  |  |
Stock offering program, maximum value | $ | $ 150,000 |  |  |  |  |  |  |  |  |  |  |
Stock Purchase Agreement | Biogen MA Inc |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Sale of stock, price per share (in dollars per share) | $ / shares |  | $ 9.2137 | $ 9.2137 |  |  |  |  |  |  |  |  |
Consideration received from sale of stock | $ |  | $ 225,000 | $ 225,000 |  |  |  |  |  |  |  |  |
Common Stock |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock, net of issuance costs (in shares) |  |  |  | 12,700,000 |  | 14,200,000 | 24,420,157 | 12,650,000 | 14,156,500 |  |  |
Public offering price of common stock issued (in dollars per share) | $ / shares |  |  |  | $ 11.50 |  | $ 16.25 |  |  |  |  |  |
Net proceeds from issuance of common stock, after deducting offering expenses | $ |  |  |  | $ 136,300 |  | $ 215,800 |  |  |  |  |  |

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