Commitments and contingencies - Narrative (Detail) $ in Millions	Mar. 31, 2022 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Decrease in purchase commitments	$ (14.4)

Purchase Obligation, Cumulative Increase (Decrease)

No definition available.

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