Pension and Other Postretirement Benefits - Funded Status (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits [Member]
Change in benefit obligation [Abstract]
Service cost	$ 0	$ 0	$ 1	$ 1
Interest cost	47	50	140	149
Change in plan assets [Roll Forward]
Contributions			150
Expected plan contributions for the remaining fiscal year			0
Other Postretirement Benefits [Member]
Change in benefit obligation [Abstract]
Service cost	0	0	0	0
Interest cost	$ 2	$ 3	$ 7	$ 8