Leases - Lease term and discount rate (Details)	Sep. 26, 2020	Dec. 31, 2019
Weighted Average Remaining Lease Term
Operating Leases	4 years 3 months 18 days	4 years 3 months 18 days
Finance Leases	7 years 3 months 18 days	8 years 4 months 24 days
Weighted Average Discount Rate
Operating Leases	3.10%	3.30%
Finance Leases	1.50%	1.40%