Retirement Plans and Other Benefits - Changes Benefit Obligations and Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Change in Benefit Obligation
Benefit obligation at the beginning of the period	$ 3,033,803	$ 3,078,648
Service cost	53,792	59,627	$ 53,080
Interest cost	131,647	123,983	129,194
Benefit payments	(142,247)	(137,115)
Actuarial (gain) loss	127,467	(91,340)
Benefit obligation at the end of the period	3,204,462	3,033,803	3,078,648
Change in Plan Assets
Balance at the beginning of the period	2,542,774	2,615,404
Actual return on plan assets	166,408	(44,690)
Employer's contributions under the plan	100,000	100,000	175,000
Benefit payments	(133,825)	(127,940)
Balance at the end of the period	2,675,357	2,542,774	2,615,404
Funded Status at the end of the period	(529,105)	(491,029)
Other Benefits
Change in Benefit Obligation
Benefit obligation at the beginning of the period	647,020	682,335
Service cost	14,993	16,827	18,139
Interest cost	29,721	28,102	41,243
Benefit payments	(26,231)	(24,988)
Actuarial (gain) loss	50,942	(55,256)
Benefit obligation at the end of the period	716,445	647,020	682,335
Change in Plan Assets
Balance at the beginning of the period	833,017	834,625
Actual return on plan assets	63,463	(2,399)
Employer's contributions under the plan	819	791	1,000
Benefit payments	(14,648)	0
Balance at the end of the period	882,651	833,017	$ 834,625
Funded Status at the end of the period	$ 166,206	$ 185,997