Retirement Plans and Other Benefits - Net Periodic Benefit Costs and Portion including Portion Charged to Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net periodic benefit costs and the portion of these costs charged to expense
Portion of cost charged to expense	$ (22,989)	$ (49,791)	$ (24,664)
Pension Benefits
Net periodic benefit costs and the portion of these costs charged to expense
Service cost-benefits earned during the period	49,902	56,669	54,858
Interest cost on benefit obligation	136,843	124,689	129,756
Expected return on plan assets	(171,884)	(182,853)	(174,271)
Amortization of prior service cost (credit)	0	0	81
Amortization of net actuarial loss	42,584	32,082	47,900
Net periodic benefit cost (benefit)	57,445	30,587	58,324
Portion of cost charged to expense	30,312	10,120	27,295
Other Benefits
Net periodic benefit costs and the portion of these costs charged to expense
Service cost-benefits earned during the period	18,369	21,100	17,119
Interest cost on benefit obligation	29,894	28,147	29,959
Expected return on plan assets	(38,412)	(42,082)	(53,401)
Amortization of prior service cost (credit)	(37,821)	(37,842)	(37,842)
Amortization of net actuarial loss	0	0	5,118
Net periodic benefit cost (benefit)	(27,970)	(30,677)	(39,047)
Portion of cost charged to expense	$ (19,859)	$ (21,426)	$ (18,274)