Pension Plans - Pension plan net periodic benefit costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 28, 2019	Jun. 29, 2018	Jun. 28, 2019	Jun. 29, 2018
Qualified Pension Plan
Defined Benefit Plan Disclosure
Service cost	$ 1,275	$ 1,224	$ 2,550	$ 2,448
Interest cost on projected benefit obligation	6,605	5,951	13,211	11,902
Expected return on plan assets	(10,640)	(11,960)	(21,280)	(23,920)
Amortization of net loss	3,815	2,843	7,630	5,686
Net pension cost (income)	1,055	(1,942)	2,111	(3,884)
SERP
Defined Benefit Plan Disclosure
Service cost	0	0	0	0
Interest cost on projected benefit obligation	59	63	118	117
Expected return on plan assets	0	0	0	0
Amortization of net loss	61	64	122	147
Net pension cost (income)	$ 120	$ 127	$ 240	$ 264