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Summary of significant accounting policies (Details)
 12 Months Ended
Dec. 31, 2017
Hardware [member]  
Disclosure Of Significant Accounting Policies [Line Items]  
Useful lives or depreciation rates, property, plant and equipment 3
Furniture and equipment [member] | Bottom of range [member]  
Disclosure Of Significant Accounting Policies [Line Items]  
Useful lives or depreciation rates, property, plant and equipment 3
Furniture and equipment [member] | Top of range [member]  
Disclosure Of Significant Accounting Policies [Line Items]  
Useful lives or depreciation rates, property, plant and equipment 5
Leasehold Improvements [Member]  
Disclosure Of Significant Accounting Policies [Line Items]  
Useful lives or depreciation rates, property, plant and equipment 3 to 15 years or up to the lease term
Other Equipments [member] | Bottom of range [member]  
Disclosure Of Significant Accounting Policies [Line Items]  
Useful lives or depreciation rates, property, plant and equipment 2
Other Equipments [member] | Top of range [member]  
Disclosure Of Significant Accounting Policies [Line Items]  
Useful lives or depreciation rates, property, plant and equipment 4