Commitments and contingencies (Detail) $ in Millions	Jun. 30, 2022 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Decrease in purchase commitments	$ 23.0

Purchase Obligation, Cumulative Increase (Decrease)

No definition available.

Name:	blue_PurchaseObligationCumulativeIncreaseDecrease
Namespace Prefix:	blue_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant