Summary of significant accounting policies - Property and Equipment (Details)	12 Months Ended
	Feb. 02, 2019
Equipment and fixtures | Minimum
Property and equipment
Estimated useful lives or the expected lease term	3 years
Equipment and fixtures | Maximum
Property and equipment
Estimated useful lives or the expected lease term	10 years
Leasehold improvements
Property and equipment
Estimated useful lives or the expected lease term	10 years
Electronic equipment and software | Minimum
Property and equipment
Estimated useful lives or the expected lease term	3 years
Electronic equipment and software | Maximum
Property and equipment
Estimated useful lives or the expected lease term	5 years