Commitments and contingencies - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended
	Jan. 31, 2020	Jun. 30, 2014
Subsequent Event
Commitments And Contingencies Disclosure [Line Items]
Contractual obligation, amendment term increase	1 year
Contractual obligation, amendment increase	$ 24.7
Pregenen
Commitments And Contingencies Disclosure [Line Items]
Contingent future cash payments		$ 120.0
Pregenen | Clinical Milestone Payments
Commitments And Contingencies Disclosure [Line Items]
Contingent future cash payments		20.1
Pregenen | Commercial Milestones Payments
Commitments And Contingencies Disclosure [Line Items]
Contingent future cash payments		$ 99.9