Goodwill and other intangible assets - Future Amortization Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	$ 2,140
Finite-Lived Intangible Assets, Amortization Expense, Year Two	1,921
Finite-Lived Intangible Assets, Amortization Expense, Year Three	1,696
Finite-Lived Intangible Assets, Amortization Expense, Year Four	1,021
Finite-Lived Intangible Assets, Amortization Expense, Year Five	841
Finite-Lived Intangible Assets, Amortization Expense, after Year Five	4,733
Finite-lived intangible assets, future amortization total	12,352
Amortization of intangible assets	12,800	$ 19,600	$ 13,100
Cost of revenue
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	1,140
Finite-Lived Intangible Assets, Amortization Expense, Year Two	1,140
Finite-Lived Intangible Assets, Amortization Expense, Year Three	915
Finite-Lived Intangible Assets, Amortization Expense, Year Four	240
Finite-Lived Intangible Assets, Amortization Expense, Year Five	60
Finite-Lived Intangible Assets, Amortization Expense, after Year Five	0
Finite-lived intangible assets, future amortization total	3,495
Operating Expense [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	1,000
Finite-Lived Intangible Assets, Amortization Expense, Year Two	781
Finite-Lived Intangible Assets, Amortization Expense, Year Three	781
Finite-Lived Intangible Assets, Amortization Expense, Year Four	781
Finite-Lived Intangible Assets, Amortization Expense, Year Five	781
Finite-Lived Intangible Assets, Amortization Expense, after Year Five	4,733
Finite-lived intangible assets, future amortization total	$ 8,857