Pension Benefits - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Apr. 01, 2018	Mar. 26, 2017
Pension Benefits
Curtailment recorded	$ 2,600
Pension Plan
Pension Benefits
Curtailment recorded	2,600
Service cost	2,647	$ 2,423
Interest cost	14,999	17,550
Expected return on plan assets	(20,555)	(26,136)
Amortization of actuarial loss	8,056	8,441
Amortization of prior service credit	(486)	(486)
Net periodic postretirement benefit cost/(income)	4,661	1,792
Qualified Plans | Pension Plan
Pension Benefits
Service cost	2,647	2,423
Interest cost	13,151	15,594
Expected return on plan assets	(20,555)	(26,136)
Amortization of actuarial loss	6,762	7,353
Amortization of prior service credit	(486)	(486)
Net periodic postretirement benefit cost/(income)	1,519	(1,252)
Pension contributions	2,100	2,000
Expected contributions in 2018	8,000
Non- Qualified Plans | Pension Plan
Pension Benefits
Service cost	0	0
Interest cost	1,848	1,956
Expected return on plan assets	0	0
Amortization of actuarial loss	1,294	1,088
Amortization of prior service credit	0	0
Net periodic postretirement benefit cost/(income)	$ 3,142	$ 3,044