Pension and Other Post-Employment Benefits (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Average Remaining Life Expectancy of Inactive Employees in Defined Benefit Plan, In Years	15 years
Accumulated Benefit Obligation	$ 310	$ 300
Estimated Net Actuarial Loss And Net Prior Service Costs For The Plans That Will Be Amortized From Accumulated Other Comprehensive Income Into Net Benefit Plan Expense	2
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	$ 2
Pension Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Average Remaining Service Period Of Active Participates In Plan In Years	7 years
Pension and Other Postretirement Benefit Expense	$ 24	24	$ 34
Defined Benefit Plan, Actual Return on Plan Assets	$ 15	9
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Average Remaining Service Period Of Active Participates In Plan In Years	13 years
Pension and Other Postretirement Benefit Expense	$ 3	$ 13	14
Domestic Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Actual Plan Asset Allocations	27.00%	26.00%
Foreign Equity[Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Actual Plan Asset Allocations	23.00%	23.00%
Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Actual Plan Asset Allocations	43.00%	44.00%
Real Estate And Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Actual Plan Asset Allocations	7.00%	7.00%
Operating Expense [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense	$ 25	$ 28	39
Administrative Expense [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense	8	9	9
Other Nonoperating Income (Expense) [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Expense	$ (6)	$ 0	$ 0