Defined Benefit Postretirement Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2018
Payment for Pension and Other Postretirement Benefits [Abstract]
Payment for Pension Benefits	$ 29
Scenario, Forecast [Member]
Payment for Pension and Other Postretirement Benefits [Abstract]
Defined Benefit Plan, Estimated Future Employer Contributions in Current Fiscal Year			$ 36
Pension Plan [Member]
Defined Benefit Plan Net Periodic Benefit Cost [Line Items]
Service cost	4	$ 6
Interest cost	7	8
Expected return on plan assets	(9)	(12)
Amortization:
- prior service cost (credit)	(2)	(2)
-actuarial loss (gain)	3	2
Net settlement loss	4	14
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	7	16
Unfunded Plan [Member]
Payment for Pension and Other Postretirement Benefits [Abstract]
Payment for Other Postretirement Benefits	8
Other Postretirement Benefit Plans Defined Benefit [Member]
Defined Benefit Plan Net Periodic Benefit Cost [Line Items]
Service cost	1	1
Interest cost	2	2
Expected return on plan assets	0	0
Amortization:
- prior service cost (credit)	(2)	(2)
-actuarial loss (gain)	0	0
Net settlement loss	0	0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	1	$ 1
Payment for Pension and Other Postretirement Benefits [Abstract]
Payment for Other Postretirement Benefits	$ 6