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, 2018	Dec. 31, 2017	Dec. 31, 2016
Net periodic benefit costs and the portion of these costs charged to expense
Portion of cost charged to expense	$ (49,791)	$ (24,664)	$ (20,373)
Pension Benefits
Net periodic benefit costs and the portion of these costs charged to expense
Service cost-benefits earned during the period	56,669	54,858	53,792
Interest cost on benefit obligation	124,689	129,756	131,647
Expected return on plan assets	(182,853)	(174,271)	(173,906)
Amortization of prior service cost (credit)	0	81	527
Amortization of net actuarial loss	32,082	47,900	40,717
Net periodic benefit cost (benefit)	30,587	58,324	52,777
Portion of cost charged to expense	10,120	27,295	26,172
Other Benefits
Net periodic benefit costs and the portion of these costs charged to expense
Service cost-benefits earned during the period	21,100	17,119	14,993
Interest cost on benefit obligation	28,147	29,959	29,721
Expected return on plan assets	(42,082)	(53,401)	(36,495)
Amortization of prior service cost (credit)	(37,842)	(37,842)	(37,883)
Amortization of net actuarial loss	0	5,118	4,589
Net periodic benefit cost (benefit)	(30,677)	(39,047)	(25,075)
Portion of cost charged to expense	$ (21,426)	$ (18,274)	$ (12,435)