Retirement Plans and Other Benefits - Changes Benefit Obligations and Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits
Change in Benefit Obligation
Benefit obligation at the beginning of the period	$ 3,078,648	$ 2,646,530
Service cost	59,627	53,080	$ 64,195
Interest cost	123,983	129,194	112,392
Benefit payments	(137,115)	(128,550)
Actuarial (gain) loss	(91,340)	378,394
Plan amendments	0	0
Benefit obligation at the end of the period	3,033,803	3,078,648	2,646,530
Change in Plan Assets
Balance at the beginning of the period	2,615,404	2,264,121
Actual return on plan assets	(44,690)	292,992
Employer's contributions under the plan	100,000	175,000	141,000
Benefit payments	(127,940)	(116,709)
Balance at the end of the period	2,542,774	2,615,404	2,264,121
Funded Status at the end of the period	(491,029)	(463,244)
Other Benefits
Change in Benefit Obligation
Benefit obligation at the beginning of the period	682,335	890,418
Service cost	16,827	18,139	23,597
Interest cost	28,102	41,243	41,536
Benefit payments	(24,988)	(29,054)
Actuarial (gain) loss	(55,256)	150,188
Plan amendments	0	(388,599)
Benefit obligation at the end of the period	647,020	682,335	890,418
Change in Plan Assets
Balance at the beginning of the period	834,625	748,339
Actual return on plan assets	(2,399)	105,223
Employer's contributions under the plan	791	770	14,000
Benefit payments	0	(19,707)
Balance at the end of the period	833,017	834,625	$ 748,339
Funded Status at the end of the period	$ 185,997	$ 152,290