Summary of significant accounting policies - Percentage accounts receivable, revenue (Details)	12 Months Ended
	Sep. 28, 2019	Sep. 29, 2018	Sep. 30, 2017
Customer A | Accounts receivable
Product Information [Line Items]
Concentration percentage	20.00%	31.00%
Customer A | Revenue
Product Information [Line Items]
Concentration percentage	16.00%	17.00%	16.00%
Customer B | Accounts receivable
Product Information [Line Items]
Concentration percentage	14.00%	13.00%
Customer C | Accounts receivable
Product Information [Line Items]
Concentration percentage		11.00%
Customer C | Revenue
Product Information [Line Items]
Concentration percentage	10.00%	10.00%	12.00%
Customer E | Accounts receivable
Product Information [Line Items]
Concentration percentage	10.00%