Pension Plans - Pension plan net periodic benefit costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Oct. 02, 2015	Sep. 30, 2016	Oct. 02, 2015
Qualified Pension Plan [Member]
Defined Benefit Plan Disclosure
Service cost	$ 1,149	$ 3,533	$ 3,447	$ 10,598
Interest cost on projected benefit obligation	6,122	6,878	18,366	20,635
Expected return on plan assets	(10,192)	(11,032)	(30,576)	(33,097)
Amortization of prior service cost	0	15	0	44
Amortization of net loss	3,173	2,479	9,520	7,438
Net pension cost	252	1,873	757	5,618
Supplemental Employee Retirement Plans [Member]
Defined Benefit Plan Disclosure
Service cost	0	52	0	155
Interest cost on projected benefit obligation	64	79	192	238
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	0	0	0	0
Amortization of net loss	45	54	136	164
Net pension cost	$ 109	$ 185	$ 328	$ 557