Commitments and Contingencies - Additional Information (Detail) $ in Millions	1 Months Ended			12 Months Ended
	May 31, 2019 USD ($)	Oct. 31, 2018 Suite	Jun. 30, 2018 USD ($) Payment Month Quarter	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2018 USD ($)
CRO [Member]
Commitments And Contingencies [Line Items]
Cancellable future commitments				$ 91.4
CRO [Member] | Other Funding Commitments [Member]
Commitments And Contingencies [Line Items]
Expenditures incurred by CROs				31.6	$ 19.6	$ 13.9
Manufacturing Agreement [Member] | Brammer [Member]
Commitments And Contingencies [Line Items]
Advance upfront fee to be paid on execution of agreements			$ 20.0
Number of non-refundable process equipment fee payments | Payment			2
Required non refundable payments at each time of procurement of equipment					$ 5.0		$ 5.0
Quarterly capacity access fee payments			$ 10.0
Manufacturing Agreement [Member] | Brammer [Member]
Commitments And Contingencies [Line Items]
Advance upfront fee to be paid on execution of agreements	$ 6.0
Quarterly capacity access fee payments	$ 13.3		$ 10.0
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
Maximum required additional quarters for remittance of capacity access fees before terminate agreement | Quarter			8
Cash payment under agreements				75.5
Manufacturing Collaboration Agreement [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
Usage fees per annum per clean room suite				1.0
Manufacturing Collaboration Agreement [Member] | Paragon [Member] | Other Noncurrent Assets [Member]
Commitments And Contingencies [Line Items]
Clean room suite reservation fees payments				40.1
Manufacturing Collaboration Agreement [Member] | Paragon [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Clean room suite reservation fees				$ 48.0
Percentage of credit on clean room use fee if threshold met				100.00%
Manufacturing Collaboration Agreement [Member]
Commitments And Contingencies [Line Items]
Pre-launch period purchase obligation, minimum amount per quarter per clean room				$ 4.0
Clinical and Commercial Supply Agreement [Member] | Aldevron LLC [Member]
Commitments And Contingencies [Line Items]
Agreement expiration date				Dec. 31, 2019
Agreement expiration description				The Company exercised the option to extend the CCSA to December 31, 2020 (the “2020 Option”) and has another option to extend the term of the CCSA for an additional year to December 31, 2021 (the “2021 Renewal Right”).
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				$ 22.8
Clinical and Commercial Supply Agreement [Member] | Aldevron LLC [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Advance upfront fee to be paid on execution of agreements				$ 20.0