Leases - Schedule of lease cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 28, 2020	Mar. 30, 2019
Lessee, Lease, Description [Line Items]
Total lease cost	$ 3,864	$ 2,640
Facilities and facilities related
Lessee, Lease, Description [Line Items]
Operating lease cost	3,585	2,452
Depreciation and amortization
Lessee, Lease, Description [Line Items]
Amortization of financing lease assets	249	163
Interest expense
Lessee, Lease, Description [Line Items]
Interest on lease liabilities	$ 30	$ 25