Commitments and Contingencies - Additional Information (Detail) $ in Millions	1 Months Ended		12 Months Ended
	Oct. 31, 2018 Suite	Jun. 30, 2018 Month	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
CRO [Member]
Commitments And Contingencies [Line Items]
Cancellable future commitments			$ 280.9
CRO [Member] | Other Funding Commitments [Member]
Commitments And Contingencies [Line Items]
Expenditures incurred by CROs			$ 40.4	$ 31.6	$ 19.6
Supply Agreement [Member] | Thermo Fisher Scientific, Inc. [Member]
Commitments And Contingencies [Line Items]
Supply agreement term		6 years
Successive renewal term of supply agreement		2 years
Minimum number of months prior to expiration of term for renewal | Month		24
Catalent Agreements [Member] | Paragon [Member]
Commitments And Contingencies [Line Items]
Number of dedicated clean room suites | Suite	2
Number of additional clean room suites available on optional reserve | Suite	2
Agreement expiration date	Dec. 31, 2024
Clinical and Commercial Supply Agreement [Member] | Aldevron LLC [Member]
Commitments And Contingencies [Line Items]
Agreement expiration date			Dec. 31, 2026
Agreement expiration description			The Company has the option to extend the term of the Aldevron Agreements by one year if the Company delivers a written notice of its intention to extend to Aldevron no later than June 1, 2025. Both parties have the right to early terminate without additional penalty. The Company has determined that the Aldevron Agreements do not contain an embedded lease because it does not convey the right to control the use of Aldevron’s facility or related equipment therein.
Percentage of prepayments credited back equal to batch invoice amount			50.00%
Clinical and Commercial Supply Agreement [Member] | Aldevron LLC [Member] | Other Current Assets [Member]
Commitments And Contingencies [Line Items]
Prepayments to extend agreement term			$ 16.3