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Long-term debt (Details) - USD ($)
$ in Millions
3 Months Ended
May 06, 2016
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Jun. 26, 2014
Long-term Debt          
Interest expense on debt   $ 0.5 $ 0.6    
Venture debt loan facility with Hercules          
Long-term Debt          
Maximum borrowing capacity $ 40.0       $ 20.0
Outstanding debt   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   20.4   $ 20.2  
Foreign currency gain   $ 0.3 $ 0.9    
Percentage of worldwide cash reserves 50.00%