Pension Benefits - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jul. 01, 2018	Apr. 01, 2018	Jun. 25, 2017	Jul. 01, 2018	Jun. 25, 2017
Pension Benefits
Curtailment recorded		$ 2,600
Pension Plan
Pension Benefits
Service cost	$ 2,553		$ 2,423	$ 5,200	$ 4,846
Interest cost	15,054		17,550	30,052	35,100
Expected return on plan assets	(20,591)		(26,136)	(41,145)	(52,272)
Amortization of actuarial loss	7,974		8,441	16,030	16,882
Amortization of prior service credit	(486)		(486)	(972)	(972)
Net periodic postretirement benefit cost/(income)	4,504		1,792	9,165	3,584
Curtailment recorded				2,600
Qualified Plans | Pension Plan
Pension Benefits
Service cost	2,553		2,423	5,200	4,846
Interest cost	13,206		15,594	26,357	31,188
Expected return on plan assets	(20,591)		(26,136)	(41,145)	(52,272)
Amortization of actuarial loss	6,680		7,353	13,442	14,706
Amortization of prior service credit	(486)		(486)	(972)	(972)
Net periodic postretirement benefit cost/(income)	1,362		(1,252)	2,882	(2,504)
Pension contributions				4,200	3,800
Expected contributions in 2018	8,000			8,000
Non- Qualified Plans | Pension Plan
Pension Benefits
Service cost	0		0	0	0
Interest cost	1,848		1,956	3,695	3,912
Expected return on plan assets	0		0	0	0
Amortization of actuarial loss	1,294		1,088	2,588	2,176
Amortization of prior service credit	0		0	0	0
Net periodic postretirement benefit cost/(income)	$ 3,142		$ 3,044	$ 6,283	$ 6,088