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Commitments and contingencies - Significant Agreements - (Details) - Collaborative Arrangement - Sanofi Genzyme
$ in Millions
1 Months Ended
Feb. 28, 2015
USD ($)
shares
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]  
Non-refundable upfront payment received $ 65.0
Redeemable Convertible Preferred Stock Series B  
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]  
Temporary equity issued (in shares) | shares 10,000,000
Temporary equity issued $ 30.0
Maximum potential in-kind services to be provided by collaborative partner $ 5.0