Retirement Plans and Other Benefits - Changes Benefit Obligations and Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Change in Benefit Obligation
Benefit obligation at the beginning of the period	$ 3,204,462	$ 3,033,803
Service cost	54,858	53,792	$ 59,627
Interest cost	129,756	131,647	123,983
Benefit payments	(166,342)	(142,247)
Actuarial (gain) loss	171,452	127,467
Benefit obligation at the end of the period	3,394,186	3,204,462	3,033,803
Change in Plan Assets
Balance at the beginning of the period	2,675,357	2,542,774
Actual return on plan assets	428,374	166,408
Employer's contributions under the plan	100,000	100,000	100,000
Benefit payments	(146,704)	(133,825)
Balance at the end of the period	3,057,027	2,675,357	2,542,774
Funded Status at the end of the period	(337,159)	(529,105)
Other Benefits
Change in Benefit Obligation
Benefit obligation at the beginning of the period	716,445	647,020
Service cost	17,119	14,993	16,827
Interest cost	29,959	29,721	28,102
Benefit payments	(30,144)	(26,231)
Actuarial (gain) loss	20,014	50,942
Benefit obligation at the end of the period	753,393	716,445	647,020
Change in Plan Assets
Balance at the beginning of the period	882,651	833,017
Actual return on plan assets	139,367	63,463
Employer's contributions under the plan	353	819	1,000
Benefit payments	0	(14,648)
Balance at the end of the period	1,022,371	882,651	$ 833,017
Funded Status at the end of the period	$ 268,978	$ 166,206