Inventory (Details) $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($)
Valoctocogene Roxaparvovec
Inventory [Line Items]
Manufacturing costs	$ 29.2

The aggregate costs incurred in the production of goods for sale.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2(a))
-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

Name:	us-gaap_ManufacturingCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration