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Commitments and contingencies - Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jan. 10, 2018
Jun. 30, 2016
Commitments and Contingencies Disclosure [Abstract]          
Operating leases, rent expense $ 2.2 $ 2.2 $ 1.8    
Funding agreement | Wellcome trust limited          
Other contingencies          
Development and regulatory milestone payments which the entity may be obligated to pay $ 22.4       $ 0.8
Subsequent Event          
Other contingencies          
Litigation settlement       $ 0.5