Employee Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2019	Sep. 30, 2018	Sep. 29, 2019	Sep. 30, 2018
Pension and SERPA Benefits:
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,072	$ 8,063	$ 19,336	$ 24,281
Interest cost	21,371	20,729	64,113	62,048
Expected return on plan assets	(35,581)	(36,925)	(106,743)	(110,742)
Amortization of unrecognized: Prior service credit	(483)	(105)	(1,449)	(316)
Amortization of unrecognized: Net loss	11,128	16,318	33,384	48,455
Settlement loss	1,500	0	1,500	0
Curtailment gain (loss)	0	0	0	1,018
Net periodic benefit cost	4,007	8,080	10,141	24,744
Postretirement Healthcare Benefits:
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,040	1,789	3,409	5,390
Interest cost	2,938	2,886	8,814	8,669
Expected return on plan assets	(3,507)	(3,541)	(10,521)	(10,623)
Amortization of unrecognized: Prior service credit	(595)	(460)	(1,785)	(1,380)
Amortization of unrecognized: Net loss	69	454	207	1,362
Special retirement benefit cost	0	0	1,583	0
Curtailment gain (loss)	0	0	(960)	0
Net periodic benefit cost	$ (55)	$ 1,128	$ 747	$ 3,418