Retirement Plans and Other Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	$ 14	$ 12	$ 30	$ 27
Interest cost on benefit obligation	31	33	62	65
Expected return on plan assets	(45)	(39)	(90)	(79)
Amortization of:
Prior service cost	0	0	0	0
Net actuarial loss	8	3	16	5
Net periodic benefit cost	8	9	18	18
Portion of cost charged to expense	5	5	11	11
Other postretirement benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	4	5	8	9
Interest cost on benefit obligation	7	11	14	23
Expected return on plan assets	(9)	(13)	(18)	(25)
Amortization of:
Prior service cost	(9)	0	(19)	0
Net actuarial loss	0	0	2	0
Net periodic benefit cost	(7)	3	(13)	7
Portion of cost charged to expense	$ (2)	$ 3	$ (4)	$ 5