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Income taxes - Narrative (Details)
$ in Millions
Dec. 31, 2018
USD ($)
Domestic Tax Authority  
Tax Credit Carryforward [Line Items]  
Domestic operating loss carryforwards $ 37.8
Foreign Tax Authority  
Tax Credit Carryforward [Line Items]  
Foreign operating loss carryforwards 14.0
Foreign Tax Authority | Research Tax Credit Carryforward  
Tax Credit Carryforward [Line Items]  
Foreign research and development tax credit carryforwards $ 2.6