Summary of significant accounting policies - Summary of changes in allowance for sales incentives (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Oct. 01, 2016
Allowance for Sales Incentives [Roll Forward]
Beginning balance	$ 11,195	$ 8,913	$ 6,235
Charged to revenue	90,246	65,879	34,627
Utilization of sales incentive allowance	(89,687)	(63,597)	(31,949)
Ending balance	$ 11,754	$ 11,195	$ 8,913