Pension Plans - Pension plan net periodic benefit costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Oct. 02, 2020	Sep. 27, 2019	Oct. 02, 2020	Sep. 27, 2019
Qualified Pension Plan
Defined Benefit Plan Disclosure
Service cost	$ 1,309	$ 1,275	$ 3,927	$ 3,825
Interest cost on projected benefit obligation	5,255	6,606	15,765	19,817
Expected return on plan assets	(10,796)	(10,640)	(32,388)	(31,920)
Amortization of net loss	1,201	3,815	3,603	11,445
Net pension (income) cost	(3,031)	1,056	(9,093)	3,167
SERP
Defined Benefit Plan Disclosure
Service cost	0	0	0	0
Interest cost on projected benefit obligation	41	59	124	177
Expected return on plan assets	0	0	0	0
Amortization of net loss	236	61	708	183
Net pension (income) cost	$ 277	$ 120	$ 832	$ 360