Defined Benefit Postretirement Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2018
Payment for Pension and Other Postretirement Benefits [Abstract]
Payment for Pension Benefits			$ 42
Scenario, Forecast [Member]
Payment for Pension and Other Postretirement Benefits [Abstract]
Defined Benefit Plan, Estimated Future Employer Contributions in Current Fiscal Year					$ 23
Pension Plan [Member]
Defined Benefit Plan Net Periodic Benefit Cost [Line Items]
Service cost	$ 5	$ 5	9	$ 11
Interest cost	6	7	13	15
Expected return on plan assets	(8)	(10)	(17)	(22)
Amortization:
- prior service cost (credit)	(3)	(2)	(5)	(4)
-actuarial loss (gain)	3	2	6	4
Net settlement loss	2	3	6	17
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	5	5	12	21
Unfunded Plan [Member]
Payment for Pension and Other Postretirement Benefits [Abstract]
Payment for Other Postretirement Benefits			12
Other Postretirement Benefit Plans Defined Benefit [Member]
Defined Benefit Plan Net Periodic Benefit Cost [Line Items]
Service cost	0	0	1	1
Interest cost	2	2	4	4
Expected return on plan assets	0	0	0	0
Amortization:
- prior service cost (credit)	(2)	(1)	(4)	(3)
-actuarial loss (gain)	1	0	1	0
Net settlement loss	0	0	0	0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	$ 1	$ 1	2	$ 2
Payment for Pension and Other Postretirement Benefits [Abstract]
Payment for Other Postretirement Benefits			$ 11