Summary of significant accounting policies - Property and equipment, net (Details)	12 Months Ended
	Sep. 29, 2018
Computer hardware and software | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	2 years
Computer hardware and software | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Furniture and fixtures | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Furniture and fixtures | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years
Tooling and production line test equipment | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	2 years
Tooling and production line test equipment | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	4 years
Leasehold improvements | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	2 years
Leasehold improvements | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	10 years
Product displays | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	1 year
Product displays | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	3 years