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Fair value measurements (Details) - Money market funds - Fair value, measurements, recurring - USD ($)
$ in Thousands
Sep. 29, 2018
Sep. 30, 2017
Assets:    
Money market funds (cash equivalents) $ 140,588 $ 40,072
Level 1    
Assets:    
Money market funds (cash equivalents) 140,588 40,072
Level 2    
Assets:    
Money market funds (cash equivalents) 0 0
Level 3    
Assets:    
Money market funds (cash equivalents) $ 0 $ 0