Retirement Plans and Other Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	$ 15	$ 13	$ 45	$ 40
Interest cost on benefit obligation	31	32	93	97
Expected return on plan assets	(45)	(39)	(134)	(119)
Amortization of:
Prior service cost	0	0	0	0
Net actuarial loss	8	3	23	8
Net periodic benefit cost	9	9	27	26
Portion of cost charged to expense	4	5	16	16
Other postretirement benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	4	5	13	14
Interest cost on benefit obligation	7	12	21	35
Expected return on plan assets	(9)	(13)	(28)	(38)
Amortization of:
Prior service cost	(9)	0	(29)	0
Net actuarial loss	1	0	4	0
Net periodic benefit cost	(6)	4	(19)	11
Portion of cost charged to expense	$ (3)	$ 3	$ (7)	$ 8