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, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Net periodic benefit costs and the portion of these costs charged to expense
Service cost-benefits earned during the period	$ 54,858	$ 53,792	$ 59,627
Interest cost on benefit obligation	129,756	131,647	123,983
Expected return on plan assets	(174,271)	(173,906)	(179,231)
Amortization of prior service cost (credit)	81	527	594
Amortization of net actuarial loss	47,900	40,717	31,056
Net periodic benefit cost	58,324	52,777	36,029
Portion of cost charged to expense	27,295	26,172	20,036
Other Benefits
Net periodic benefit costs and the portion of these costs charged to expense
Service cost-benefits earned during the period	17,119	14,993	16,827
Interest cost on benefit obligation	29,959	29,721	28,102
Expected return on plan assets	(53,401)	(36,495)	(36,855)
Amortization of prior service cost (credit)	(37,842)	(37,883)	(37,968)
Amortization of net actuarial loss	5,118	4,589	4,881
Net periodic benefit cost	(39,047)	(25,075)	(25,013)
Portion of cost charged to expense	$ (18,274)	$ (12,435)	$ (10,391)