Pension and Other Postretirement Benefits - Pension Benefits - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Apr. 01, 2018	Sep. 24, 2017	Sep. 30, 2018	Sep. 24, 2017
Pension Benefits
Curtailment recorded		$ 2,600
Gain on multiemployer pension plan withdrawal liability	$ 4,851		$ 0	$ 4,851	$ 0
Pension Plan
Pension Benefits
Service cost	2,393		2,423	7,593	7,269
Interest cost	15,055		17,552	45,107	52,652
Expected return on plan assets	(20,591)		(26,136)	(61,736)	(78,408)
Amortization of actuarial loss	7,974		8,439	24,004	25,321
Amortization of prior service credit	(487)		(486)	(1,459)	(1,458)
Other	421		0	421	0
Net periodic postretirement benefit income	4,765		1,792	13,930	5,376
Curtailment recorded				2,600
Gain on multiemployer pension plan withdrawal liability	4,900
Qualified Plans | Pension Plan
Pension Benefits
Service cost	2,393		2,423	7,593	7,269
Interest cost	13,207		15,596	39,564	46,784
Expected return on plan assets	(20,591)		(26,136)	(61,736)	(78,408)
Amortization of actuarial loss	6,680		7,351	20,122	22,057
Amortization of prior service credit	(487)		(486)	(1,459)	(1,458)
Other	0		0	0	0
Net periodic postretirement benefit income	1,202		(1,252)	4,084	(3,756)
Pension contributions				6,200	5,900
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	5,543	5,868
Expected return on plan assets	0		0	0	0
Amortization of actuarial loss	1,294		1,088	3,882	3,264
Amortization of prior service credit	0		0	0	0
Other	421		0	421	0
Net periodic postretirement benefit income	$ 3,563		$ 3,044	$ 9,846	$ 9,132