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Stock-based compensation - Additional Information (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Dec. 28, 2019
Sep. 28, 2019
Mar. 28, 2020
Sep. 28, 2019
Stock Options        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period     48 months  
Cliff vesting period     1 year  
Unrecognized stock-based compensation expense   $ 43.9 $ 29.9 $ 43.9
Unrecognized stock-based compensation expense, period of recognition     1 year 8 months 12 days 2 years
Restricted Stock Units        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Cliff vesting period     4 years  
Unrecognized stock-based compensation expense   $ 55.6 $ 62.6 $ 55.6
Unrecognized stock-based compensation expense, period of recognition 3 years 1 month 6 days 3 years 4 months 24 days    
2003 Stock Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Exercisable period     10 years