Commitments and contingencies - Additional Information (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($)
Commitments And Contingencies Disclosure [Line Items]
Decrease in expected future minimum purchase commitments	$ 89.2
Pregenen
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments	120.0
Pregenen | Clinical Milestone Payments
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments	20.1
Pregenen | Commercial Milestones Payments
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments	$ 99.9