Pension Plan - Pension plan net periodic benefit costs (Details) - USD ($) $ in Thousands	3 Months Ended
	Apr. 01, 2016	Apr. 03, 2015
Qualified Pension Plan [Member]
Defined Benefit Plan Disclosure
Service cost	$ 1,149	$ 3,533
Interest cost on projected benefit obligation	6,122	6,878
Expected return on plan assets	(10,192)	(11,032)
Amortization of prior service cost (credit)	0	14
Recognized net loss	3,173	2,480
Net pension benefit cost	252	1,873
Supplemental Employee Retirement Plans [Member]
Defined Benefit Plan Disclosure
Service cost	0	52
Interest cost on projected benefit obligation	64	79
Expected return on plan assets	0	0
Amortization of prior service cost (credit)	0	0
Recognized net loss	45	55
Net pension benefit cost	$ 109	$ 186