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Stock-based compensation - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 12 Months Ended
Jul. 31, 2018
Sep. 28, 2019
Sep. 29, 2018
Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock options granted, fair value   $ 8.4 $ 50.2 $ 39.4
Weighted-average grant date fair value (in dollars per share)   $ 4.91 $ 5.39 $ 4.84
Fair value of options vested   $ 34.3 $ 38.3 $ 34.8
Intrinsic value of stock options exercised   61.1 31.3 $ 42.0
Unrecognized stock-based compensation expense   $ 43.9 $ 71.5  
Stock options to purchase common stock        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period   48 months    
Unrecognized stock-based compensation expense, period of recognition   2 years 2 years 7 months 6 days  
RSUs        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period   4 years    
Unrecognized stock-based compensation expense, period of recognition   3 years 5 months    
Unrecognized stock-based compensation expense   $ 55.6    
2003 Stock Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares reserved for future issuance (in shares) 21,839,258      
Outstanding shares increase for future issuance 5.00%      
2018 Equity Incentive Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares reserved for future issuance (in shares)   25,280,393    
Exercisable period   10 years