Retirement Plans and Other Benefits - Changes Benefit Obligations and Funded Status (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Benefits
Change in Benefit Obligation
Benefit obligation at the beginning of the period	$ 2,646,530	$ 2,850,846
Service cost	53,080	64,195	63,502
Interest cost	129,194	112,392	119,586
Benefit payments	(128,550)	(125,269)
Actuarial (gain) loss	378,394	(255,634)
Plan amendments	0	0
Benefit obligation at the end of the period	3,078,648	2,646,530	2,850,846
Change in Plan Assets
Balance at the beginning of the period	2,264,121	2,079,181
Actual return on plan assets	292,992	150,546
Employer's contributions under the plan	175,000	140,500	65,000
Benefit payments	(116,709)	(106,106)
Balance at the end of the period	2,615,404	2,264,121	2,079,181
Funded Status at the end of the period	(463,244)	(382,409)
Other Benefits
Change in Benefit Obligation
Benefit obligation at the beginning of the period	890,418	990,418
Service cost	18,139	23,597	27,163
Interest cost	41,243	41,536	46,467
Benefit payments	(29,054)	(26,675)
Actuarial (gain) loss	150,188	(138,458)
Plan amendments	(388,599)	0
Benefit obligation at the end of the period	682,335	890,418	990,418
Change in Plan Assets
Balance at the beginning of the period	748,339	684,221
Actual return on plan assets	105,223	76,995
Employer's contributions under the plan	770	14,438	22,707
Benefit payments	(19,707)	(27,315)
Balance at the end of the period	834,625	748,339	684,221
Funded Status at the end of the period	$ 152,290	$ (142,079)