Leases - Schedule of lease cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 27, 2020	Jun. 29, 2019	Jun. 27, 2020	Jun. 29, 2019
Lessee, Lease, Description [Line Items]
Total lease cost	$ 3,806	$ 2,971	$ 7,670	$ 5,611
Facilities and facilities related
Lessee, Lease, Description [Line Items]
Operating lease cost	3,515	2,790	7,100	5,242
Depreciation and amortization
Lessee, Lease, Description [Line Items]
Amortization of financing lease assets	260	161	509	324
Interest expense
Lessee, Lease, Description [Line Items]
Interest on lease liabilities	$ 31	$ 20	$ 61	$ 45