Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Other comprehensive income (loss) tax expense	$ 0.4	$ 0.0

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

No definition available.

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