Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Regulatory Liabilities	$ 1,391	$ 1,319
Capital Lease Obligations Incurred	50	(5)	$ (2)
Contract with Customer, Liability, Cumulative Catch-up Adjustment to Revenue, Change in Estimate of Transaction Price	3	0	0
Total Revenues	$ 1,991	2,009	1,923
Business days after the invoice date	8
Other Investments	$ 112	30
Days after the due date	45
Wholesale accounts receivable written off	$ 0	0	$ 0
Margin deposit	16	11
Letters of Credit Outstanding, Amount	$ 48	$ 31
Public Utilities, Allowance for Funds Used During Construction, Rate	7.30%	7.30%	7.30%
Public Utilities, Allowance for Funds Used During Construction, Capitalized Interest	$ 6	$ 6	$ 11
Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity	$ 11	$ 12	$ 21
Depreciation expense rate	3.60%	3.60%	3.50%
Finite-Lived Intangible Assets, Accumulated Amortization	$ 302	$ 296
Amortization of Intangible Assets	59	$ 46	$ 44
Future Amortization Expense, Year One	60
Future Amortization Expense, Year Two	52
Future Amortization Expense, Year Three	44
Future Amortization Expense, Year Four	38
Future Amortization Expense, Year Five	29
Power Cost Deadband - Lower Threshold	15
Power Cost Deadband - Upper Threshold	$ 30
Public Utilities, Approved Return on Equity, Percentage	9.50%	9.60%	9.60%
Regulatory Liability, Noncurrent	$ 1,355	$ 1,288
Franchise taxes	45	43	$ 43
Regulatory Assets, Noncurrent	401	438
Pension and Other Postretirement Benefits Cost (Reversal of Cost)	30	24	28
Proceeds from (Payments for) Other Financing Activities	(5)	(7)	(16)
Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	131
Pension and Other Postretirement Plans Costs [Member]
Property, Plant and Equipment [Line Items]
Regulatory Assets	3
Environmental Restoration Costs [Member]
Property, Plant and Equipment [Line Items]
Regulatory Assets, Noncurrent	25
Asset Retirement Obligation Costs [Member]
Property, Plant and Equipment [Line Items]
Regulatory Liabilities	53	52
Deferred Income Tax Charge [Member]
Property, Plant and Equipment [Line Items]
Regulatory Liabilities	267	277
Environmental Restoration Costs [Member]
Property, Plant and Equipment [Line Items]
Regulatory Liabilities	1	3
Regulatory Liability, Noncurrent		3
Minimum [Member]
Property, Plant and Equipment [Line Items]
Finance Lease, Right-of-Use Asset	40
Maximum [Member]
Property, Plant and Equipment [Line Items]
Finance Lease, Right-of-Use Asset	50
Debt [Member]
Property, Plant and Equipment [Line Items]
Proceeds from (Payments for) Other Financing Activities			9
Restatement Adjustment [Member]
Property, Plant and Equipment [Line Items]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)	$ 5	$ 4	$ 7