Pension Benefits - Net Periodic Pension Cost, AOCI (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 28, 2020	Mar. 30, 2019
Components of net periodic pension cost
Service cost-benefits earned during the period	$ 2,467	$ 1,937
Interest cost on projected benefit obligation	1,377	1,442
Expected return on plan assets	(2,229)	(1,914)
Amortization of unrecognized loss	400	215
Net periodic pension cost	$ 2,015	$ 1,680