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Stock-based compensation - Restricted Stock Unit Activity (Details) - Restricted Stock Units - USD ($)
$ / shares in Units, $ in Thousands
 6 Months Ended
Mar. 28, 2020
Sep. 28, 2019
Number of Units    
Outstanding, beginning balance (in shares) 6,716,786  
Forfeited (in shares) 1,572,617  
Exercised (in shares) (923,636)  
Granted (in shares) (382,429)  
Outstanding, ending balance (in shares) 6,983,338  
Weighted Average Grant Date Fair Value    
Outstanding, beginning balance (in usd per share) $ 11.40  
Outstanding, ending balance (in usd per share) $ 11.99  
Additional Information    
Aggregate Intrinsic Value $ 60,196 $ 90,744
Options vested and expected to vest (in shares) 5,648,433  
Options vested and expected to vest - Weighted Average Exercise Price (in USD per share) $ 12.00  
Options vested and expected to vest - Weighted Average Intrinsic Value $ 48,689