Stock-based compensation - Stock-based compensation expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 28, 2019	Sep. 29, 2018	Sep. 30, 2017
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 46,575	$ 38,645	$ 36,550
Cost of revenue
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	985	198	240
Research and development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	17,643	13,960	13,605
Sales and marketing
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	12,965	15,885	15,086
General and administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 14,982	$ 8,602	$ 7,619