Commitments and contingencies - Additional Information (Detail) - Pregenen [Member] $ in Millions	Mar. 31, 2019 USD ($)
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments	$ 120.0
Clinical Milestone Payments [Member]
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments	20.1
Commercial Milestones Payments [Member]
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments	$ 99.9

For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

Name:	us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant