Defined Benefit Postretirement Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2017
Payment for Pension and Other Postretirement Benefits [Abstract]
Payment for Pension Benefits			$ 45
Scenario, Forecast [Member]
Payment for Pension and Other Postretirement Benefits [Abstract]
Defined Benefit Plan, Estimated Future Employer Contributions in Current Fiscal Year					$ 8
Pension Plan [Member]
Defined Benefit Plan Net Periodic Benefit Cost [Line Items]
Service cost	$ 5	$ 6	16	$ 18
Interest cost	7	9	22	30
Expected return on plan assets	(10)	(12)	(32)	(40)
Amortization:
- prior service cost (credit)	(3)	(2)	(7)	(7)
-actuarial loss (gain)	3	4	7	11
Net settlement loss	8	14	25	93
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	10	19	31	105
Unfunded Plan [Member]
Payment for Pension and Other Postretirement Benefits [Abstract]
Payment for Other Postretirement Benefits			10
Other Postretirement Benefit Plans Defined Benefit [Member]
Defined Benefit Plan Net Periodic Benefit Cost [Line Items]
Service cost	0	1	1	3
Interest cost	2	3	6	8
Expected return on plan assets	0	0	0	0
Amortization:
- prior service cost (credit)	(2)	(1)	(5)	(3)
-actuarial loss (gain)	0	0	0	0
Net settlement loss	0	0	0	0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	$ 0	$ 3	2	$ 8
Payment for Pension and Other Postretirement Benefits [Abstract]
Payment for Other Postretirement Benefits			$ 16