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Summary of significant accounting policies - Narrative (Details) - USD ($)
$ in Thousands
 9 Months Ended
Jun. 27, 2020
Sep. 29, 2019
Sep. 28, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Accounting standards update [Extensible List] us-gaap:AccountingStandardsUpdate201602Member    
Operating lease right-of-use assets $ 43,325    
Operating lease liabilities 64,643    
Assets 644,628   $ 761,605
Liabilities $ 383,429   $ 480,677
Accounting Standards Update 2016-02      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Operating lease right-of-use assets   $ 63,700  
Operating lease liabilities   73,700  
Assets   62,500  
Liabilities   $ 62,500