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Summary of significant accounting policies - Schedule of Useful Lives by Major Asset Class (Details)
12 Months Ended
Dec. 31, 2018
Machinery, equipment and software | Minimum  
Property, Plant and Equipment [Line Items]  
Estimated useful lives 3 years
Machinery, equipment and software | Maximum  
Property, Plant and Equipment [Line Items]  
Estimated useful lives 5 years
Leasehold improvements  
Property, Plant and Equipment [Line Items]  
Estimated useful lives 5 years
Furniture and fixtures | Minimum  
Property, Plant and Equipment [Line Items]  
Estimated useful lives 2 years
Furniture and fixtures | Maximum  
Property, Plant and Equipment [Line Items]  
Estimated useful lives 5 years
Store fixtures | Minimum  
Property, Plant and Equipment [Line Items]  
Estimated useful lives 2 years
Store fixtures | Maximum  
Property, Plant and Equipment [Line Items]  
Estimated useful lives 3 years