Pension and Other Postretirement Benefits - Funded Status (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits [Member]
Change in benefit obligation [Abstract]
Service cost	$ 0	$ 1	$ 1	$ 1
Interest cost	42	45	84	93
Pension Benefits [Member] | United States [Member] | Qualified Plan [Member]
Change in plan assets [Roll Forward]
Contributions			0
Expected plan contributions for the remaining fiscal year	0		0
Other Postretirement Benefits [Member]
Change in benefit obligation [Abstract]
Service cost	0	0	0	0
Interest cost	$ 2	$ 2	$ 4	$ 5