Pension and Other Postretirement Benefits - Funded Status (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits [Member]
Change in benefit obligation [Abstract]
Service cost	$ 1	$ 1	$ 1	$ 1
Interest cost	45	49	93	99
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	$ 2	$ 5	$ 5