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Commitments and Contingencies - Additional Information (Detail) |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
May 03, 2021 | Sep. 30, 2021 USD ($) | Sep. 30, 2020 USD ($) | Sep. 30, 2021 USD ($) ft² OfficeSpace Milestone | Sep. 30, 2020 USD ($) | Dec. 31, 2020 Milestone | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2015 USD ($)
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |
Rental expense |  | $ 63,000 | $ 50,000 | $ 189,000 | $ 143,000 |  |  |  |
Debt outstanding |  | 0 |  | 0 |  |  |  |  |
Indemnification obligations amount |  | 0 |  | 0 |  |  |  |  |
Contingent liabilities |  | 0 |  | $ 0 |  |  |  |  |
Office of Chief Scientist of Israeli Ministry of Economy and Industry [Member] |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |
Grants received |  |  |  |  |  |  |  |  | $ 500,000
ALS Therapy Development Foundation, Inc. License Agreement [Member] |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |
Fee due for milestones achieved |  |  |  |  |  |  | $ 1,000,000.0 | $ 1,000,000.0 |
Common stock issued in lieu of making a cash payment |  |  |  |  |  |  | $ 1,000,000.0 | $ 1,000,000.0 |
Number of Milestones Achieved | Milestone |  |  |  | 0 |  | 0 |  |  |
Remaining milestone payments for first licensed product |  | 6,000,000.0 |  | $ 6,000,000.0 |  |  |  |  |
Annual license maintenance fee |  |  |  | 100,000 |  |  |  |  |
ALS Therapy Development Foundation, Inc. License Agreement [Member] | Achievement of 500 Million Aggregate Sales [Member] |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |
Reaching of aggregate net sales |  |  |  | 500,000,000.0 |  |  |  |  |
Amount of one-time milestone payment |  | 15,000,000.0 |  | 15,000,000.0 |  |  |  |  |
ALS Therapy Development Foundation, Inc. License Agreement [Member] | Achievement of 1 Billion Aggregate Sales [Member] |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |
Reaching of aggregate net sales |  |  |  | 1,000,000,000.0 |  |  |  |  |
Amount of one-time milestone payment |  | 30,000,000.0 |  | $ 30,000,000.0 |  |  |  |  |
Minimum [Member] |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |
Remaining term of office lease |  |  |  | 1 month |  |  |  |  |
Maximum [Member] |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |
Remaining term of office lease |  |  |  | 18 months |  |  |  |  |
Maximum [Member] | ALS Therapy Development Foundation, Inc. License Agreement [Member] |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |
Development and regulatory milestone payments |  | $ 2,500,000 |  | $ 2,500,000 |  |  |  |  |
Irvine, California [Member] |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |
Area of office space | ft² |  |  |  | 5,197 |  |  |  |  |
Lease expiration date | Dec. 31, 2022 |  |  | Sep. 30, 2021 |  |  |  |  |
Effective date of lease amendment | Oct. 01, 2021 |  |  |  |  |  |  |  |
Burlington, Massachusetts [Member] |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |
Lease expiration date |  |  |  | Jun. 30, 2021 |  |  |  |  |
Lease expired term |  |  |  | Company had operating leases for four serviced office spaces in Burlington, Massachusetts that expired on June 30, 2021, which have been converted to monthly leases |  |  |  |  |
Number of office spaces operating lease | OfficeSpace |  |  |  | 4 |  |  |  |  |

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