Pension Benefits - Net Periodic Pension Cost, AOCI (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 27, 2020	Jun. 29, 2019	Jun. 27, 2020	Jun. 29, 2019
Components of net periodic pension cost
Service cost-benefits earned during the period	$ 2,052	$ 1,734	$ 4,519	$ 3,671
Interest cost on projected benefit obligation	1,322	1,431	2,699	2,873
Expected return on plan assets	(2,319)	(1,913)	(4,548)	(3,827)
Amortization of unrecognized loss	296	215	696	430
Net periodic pension cost	$ 1,351	$ 1,467	$ 3,366	$ 3,147