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Summary of significant accounting policies - Summary of changes in allowance for doubtful accounts (Details) - USD ($)
$ in Thousands
12 Months Ended
Sep. 29, 2018
Sep. 30, 2017
Oct. 01, 2016
Allowance for Doubtful Accounts Receivable [Roll Forward]      
Beginning balance $ 804 $ 726 $ 679
Increases 635 449 962
Write-offs (567) (371) (915)
Ending balance $ 872 $ 804 $ 726