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, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Net periodic benefit costs and the portion of these costs charged to expense
Service cost-benefits earned during the period	$ 53,792	$ 59,627	$ 53,080
Interest cost on benefit obligation	131,647	123,983	129,194
Expected return on plan assets	(173,906)	(179,231)	(158,998)
Amortization of prior service cost (credit)	527	594	869
Amortization of net actuarial loss	40,717	31,056	10,963
Net periodic benefit cost	52,777	36,029	35,108
Portion of cost charged to expense	26,172	20,036	21,985
Other Benefits
Net periodic benefit costs and the portion of these costs charged to expense
Service cost-benefits earned during the period	14,993	16,827	18,139
Interest cost on benefit obligation	29,721	28,102	41,243
Expected return on plan assets	(36,495)	(36,855)	(46,400)
Amortization of prior service cost (credit)	(37,883)	(37,968)	(9,626)
Amortization of net actuarial loss	4,589	4,881	1,175
Net periodic benefit cost	(25,075)	(25,013)	4,531
Portion of cost charged to expense	$ (12,435)	$ (10,391)	$ 6,000