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Revenue recognition - Recognized revenue in the period (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Revenue from Contract with Customer [Abstract]    
Amounts included in contract liabilities at the beginning of the period $ 0  
Performance obligations satisfied in previous period 0  
Performance obligations satisfied in current period 55,981  
Total product revenue $ 55,981 $ 26,442