Employee Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Apr. 01, 2018	Jun. 30, 2019	Jul. 01, 2018
Pension and SERPA Benefits:
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,632	$ 8,063	$ 13,264	$ 16,218
Interest cost	21,371	20,729	42,742	41,319
Expected return on plan assets	(35,581)	(36,926)	(71,162)	(73,817)
Amortization of unrecognized: Prior service credit	(483)	(105)	(966)	(211)
Amortization of unrecognized: Net loss	11,128	16,318	22,256	32,137
Curtailment (gain) loss	0	0	0	1,018
Net periodic benefit cost	3,067	8,079	6,134	16,664
Postretirement Healthcare Benefits:
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,185	1,789	2,369	3,601
Interest cost	2,938	2,886	5,876	5,783
Expected return on plan assets	(3,507)	(3,541)	(7,014)	(7,082)
Amortization of unrecognized: Prior service credit	(595)	(460)	(1,190)	(920)
Amortization of unrecognized: Net loss	69	454	138	908
Special retirement benefits	1,583	0	1,583	0
Curtailment (gain) loss	(960)	0	(960)	0
Net periodic benefit cost	$ 713	$ 1,128	$ 802	$ 2,290