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STOCKHOLDERS' EQUITY - Narrative (Details) - USD ($) |  | 1 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
May 31, 2022 | Dec. 31, 2022 | May 31, 2021 | Aug. 31, 2020 | Apr. 30, 2020 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Jun. 30, 2023 | Jun. 30, 2020 | Jun. 29, 2020 | May 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, shares authorized (in shares) |  | 5,000,000 |  |  |  | 5,000,000 | 5,000,000 |  |  |  |  |
Preferred stock, shares issued (in shares) |  | 0 |  |  |  | 0 | 0 |  |  |  |  |
Preferred stock, shares outstanding (in shares) |  | 0 |  |  |  | 0 | 0 |  |  |  |  |
Common stock, shares authorized (in shares) |  | 640,000,000 |  |  |  | 640,000,000 | 640,000,000 |  | 640,000,000 | 320,000,000 | 160,000,000 |
Common stock, shares outstanding (in shares) |  | 166,793,320 |  |  |  | 178,133,548 | 166,793,320 |  |  |  |  |
Stock option maximum term |  |  |  |  |  | 4 years 7 months 9 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% |  |  |  |  |  |  |  |  |  |  |
Shares reserved for issuance of future awards (in shares) |  |  |  |  |  | 2,858,653 |  |  |  |  |  |
Intrinsic value of options exercised |  |  |  |  |  | $ 0 | $ 0 | $ 2,800,000 |  |  |  |
Restricted stock units awarded (in shares) |  |  |  |  |  | 4,811,834 |  |  |  |  |  |
Total payments for the employees' tax obligations to taxing authorities |  |  |  |  |  | $ 1,453,000 | $ 2,104,000 | $ 3,258,000 |  |  |  |
Restricted Stock Units (RSUs) |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock options vesting period |  |  |  |  |  | 3 years |  |  |  |  |  |
Restricted stock units awarded (in shares) |  |  |  |  |  | 4,811,834 | 4,349,795 | 2,140,785 |  |  |  |
Grant date fair value per award of restricted stock units (in dollars per share) |  |  |  |  |  | $ 2.29 | $ 5.52 | $ 11.16 |  |  |  |
Aggregate fair value of RSUs vested in period |  |  |  |  |  | $ 17,800,000 | $ 13,100,000 | $ 9,000,000 |  |  |  |
Shares withheld from issuance in order to pay employee taxes (in shares) |  |  |  |  |  | 788,236 | 380,917 | 293,120 |  |  |  |
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) | 7,900,000 |  |  |  |  |  |  |  | 10,000,000 |  |  | 9,900,000
2018 Stock Incentive 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 | 100.00% |  |  |  |  |  |  |  |  |  |  |
Stock options expiration term | 10 years |  |  |  |  |  |  |  |  |  |  |
Fixed ratio shares of common stock | 1.33 |  |  |  |  |  |  |  |  |  |  |
Shares reserved for issuance of future awards (in shares) |  |  |  |  |  | 13,738,867 |  |  |  |  |  |
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) |  |  | 5,000,000 |  |  |  |  |  |  |  |  |
Purchase plan offering period |  |  | 2 years |  |  |  |  |  |  |  |  |
Purchase plan purchase period |  |  | 6 months |  |  |  |  |  |  |  |  |
Maximum | 2018 Stock Incentive Plan |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock option maximum term | 10 years |  |  |  |  |  |  |  |  |  |  |
Stock options vesting period | 4 years |  |  |  |  |  |  |  |  |  |  |
Minimum | 2018 Stock Incentive Plan |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock options vesting period | 3 years |  |  |  |  |  |  |  |  |  |  |
Jefferies LLC | ATM Agreement |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock offering program, maximum value |  |  |  | $ 150,000,000 |  |  |  |  |  |  |  |
Stock offering program, increase to aggregate offering price |  | $ 175,000,000 |  |  |  |  |  |  |  |  |  |
Amount remained available |  |  |  |  |  | $ 194,500,000 |  |  |  |  |  |
Issuance of common stock in at-the-market offering, net of offering expenses (in shares) |  |  |  |  |  | 8,249,261 | 19,300,743 |  |  |  |  |
Consideration received from sale of stock |  |  |  |  |  | $ 15,100,000 | $ 84,900,000 |  |  |  |  |
Stock Purchase Agreement | Biogen MA, Inc. |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Consideration received from sale of stock |  |  |  |  | $ 225,000,000 |  |  |  |  |  |  |

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