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Stock-based compensation - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Sep. 29, 2018
Sep. 30, 2017
Oct. 01, 2016
Jul. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock options granted, fair value $ 50.2 $ 39.4 $ 83.5  
Weighted-average grant date fair value (in dollars per share) $ 5.39 $ 4.84 $ 5.32  
Intrinsic value of stock options exercised $ 31.3 $ 42.0 $ 17.4  
Unrecognized stock-based compensation expense $ 71.5 $ 74.9    
2003 Stock Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period 48 months      
Exercisable period 10 years      
2018 Equity Incentive Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares authorized       21,200,000
Shares reserved for future issuance       21,939,408
Stock options to purchase common stock        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Unrecognized stock-based compensation expense, period of recognition 2 years 7 months 6 days 2 years 9 months 20 days