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Long-term debt (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
May 06, 2016
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Long-term Debt            
Foreign currency gain       $ 2.3    
Foreign currency loss     $ 0.2   $ 0.4  
Interest expense on debt   $ 0.6 $ 0.5 1.6 $ 1.7  
Venture debt loan facility with Hercules            
Long-term Debt            
Outstanding debt   20.0   20.0   $ 20.0
Interest rate (as a percent) 8.25%          
Discount rate (as a percent) 5.25%          
Back-end fee (as a percent) 4.85%          
Facility fee (as a percent) 0.75%          
Amount of first cumulative raisings $ 30.0          
Amount of second cumulative raisings $ 50.0          
Amortized cost net of discount and debt issuance costs   20.6   $ 20.6   $ 20.2
Foreign currency gain   $ 0.7        
Percentage of worldwide cash reserves 50.00%