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Stockholders' equity - Textual information (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Aug. 06, 2018
Sep. 29, 2018
Sep. 30, 2017
Oct. 01, 2016
Equity [Abstract]        
Common shares issued in conversion of outstanding shares of redeemable convertible preferred stock (shares) 32,482,590      
Number of shares of common stock issued for each share of redeemable convertible preferred stock (shares) 1   32,482,590  
Common stock, authorized (in shares) 500,000,000 500,000,000 151,458,824  
Common stock, par value (in dollars per share) $ 0.001 $ 0.001 $ 0.001  
Preferred stock, authorized (in shares) 10,000,000      
Preferred stock, par value (in dollars per share) $ 0.001      
Class of Stock [Line Items]        
Repurchase of common stock   $ 911 $ 10,016 $ 145
Private placement        
Class of Stock [Line Items]        
Repurchase of common stock (in shares)     738,682  
Repurchase of common stock     $ 10,000  
Issuance of common, net of issuance costs (in shares)     742,034  
Sale of stock, price per share (in dollars per share)     $ 13.56  
Proceeds from sale of stock     $ 10,100