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Fair value measurements (Details) - Money market funds - USD ($)
$ in Thousands
Dec. 29, 2018
Sep. 29, 2018
Assets:    
Money market funds (cash equivalents) $ 100,111 $ 140,588
Level 1    
Assets:    
Money market funds (cash equivalents) 100,111 140,588
Level 2    
Assets:    
Money market funds (cash equivalents) 0 0
Level 3    
Assets:    
Money market funds (cash equivalents) $ 0 $ 0