Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Other comprehensive income (loss) tax expense	$ 0.8	$ 0.0	$ 1.3	$ 0.0

Other comprehensive income loss unrealized gain loss on securities and foreign currency translation adjustments arising during period tax.

No definition available.

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Namespace Prefix:	blue_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration