Pension Plans - Pension plan net periodic benefit costs (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 30, 2018	Mar. 31, 2017
Qualified Pension Plan
Defined Benefit Plan Disclosure
Service cost	$ 1,224	$ 1,198
Interest cost on projected benefit obligation	5,951	6,089
Expected return on plan assets	(11,960)	(10,512)
Amortization of net loss	2,843	3,486
Additional amount recognized due to curtailment/settlement	0	0
Net pension cost	(1,942)	261
SERP
Defined Benefit Plan Disclosure
Service cost	0	0
Interest cost on projected benefit obligation	54	61
Expected return on plan assets	0	0
Amortization of net loss	83	58
Additional amount recognized due to curtailment/settlement	0	99
Net pension cost	$ 137	$ 218