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Commitments and Contingencies - Additional Information (Detail) $ in Millions |  | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jan. 17, 2018 USD ($) ft² Option | Jul. 31, 2016 USD ($) | Aug. 31, 2015 USD ($) | Mar. 31, 2018 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Other Commitments [Line Items] |  |  |  |  |  |  |  |
Rent expense |  |  |  | $ 0.2 | $ 0.2 | $ 0.8 | $ 0.4 | $ 0.1
Office Space [Member] |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |
Operating lease beginning date |  |  | 2016-01 |  |  |  |  |
Operating lease expiration date |  |  | 2020-12 |  |  |  |  |
Annual lease payments |  |  | $ 0.4 |  |  |  |  |
Term of lease |  |  | 5 years |  |  |  |  |
Lease renewal term |  |  | 5 years |  |  |  |  |
Security deposit |  |  | $ 0.2 |  |  |  |  |
Annual rent escalations and tenant incentives |  |  | 0.7 |  |  |  |  |
Amount reimbursed |  |  | $ 0.3 |  |  |  |  |
Office Space [Member] | Lease Agreements [Member] | Cambridge, Massachusetts [Member] | U.S. REIF Central Plaza Massachusetts, LLC [Memebr] |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |
Term of lease | 7 years |  |  |  |  |  |  |
Lease renewal term | 5 years |  |  |  |  |  |  |
Lease expiration date | Dec. 01, 2018 |  |  |  |  |  |  |
Area of office space | ft² | 7,800 |  |  |  |  |  |  |
Operating lease, option to extend | the Company has two consecutive options to extend the Lease Term for an additional period of five years (the “Option Terms”), subject to certain conditions, upon notice to the Landlord. |  |  | the Company has two consecutive options to extend the Lease Term for an additional period of five years (the “Option Terms”), subject to certain conditions, upon notice to the Landlord. |  |  |  |
Number of renewal options | Option | 2 |  |  |  |  |  |  |
Operating leases annual base rent initial | $ 0.5 |  |  |  |  |  |  |
Operating leases annual base rent final | $ 0.6 |  |  |  |  |  |  |
Operating lease percentage of annual base rent | 100.00% |  |  |  |  |  |  |
Leasehold improvements receivable from the landlord | $ 0.4 |  |  |  |  |  |  |
Laboratory Space [Member] |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |
Annual lease payments |  | $ 0.3 |  |  |  |  |  |
Lease expiration date |  |  |  | Nov. 30, 2019 |  |  |  |

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