Summary of significant accounting policies - Narrative (Details) - USD ($) $ in Thousands	Dec. 28, 2019	Sep. 29, 2019	Sep. 28, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use assets	$ 60,007
Operating lease liabilities, noncurrent	69,382
Assets	873,929		$ 761,605
Liabilities	$ 506,650		$ 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, noncurrent		73,700
Assets		62,500
Liabilities		$ 62,500