Pension Plans - Pension plan net periodic benefit costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 29, 2018	Jun. 30, 2017	Jun. 29, 2018	Jun. 30, 2017
Qualified Pension Plan
Defined Benefit Plan Disclosure
Service cost	$ 1,224	$ 1,199	$ 2,448	$ 2,397
Interest cost on projected benefit obligation	5,951	6,090	11,902	12,179
Expected return on plan assets	(11,960)	(10,512)	(23,920)	(21,024)
Amortization of net loss	2,843	3,486	5,686	6,972
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment			0	0
Net pension (income) cost	(1,942)	263	(3,884)	524
SERP
Defined Benefit Plan Disclosure
Service cost	0	0	0	0
Interest cost on projected benefit obligation	63	62	117	123
Expected return on plan assets	0	0	0	0
Amortization of net loss	64	8	147	66
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment			0	99
Net pension (income) cost	$ 127	$ 70	$ 264	$ 288