Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2016
Property, Plant and Equipment [Line Items]
Unamortized Debt Issuance Expense	$ 11	$ 11
Other Investments	$ 1	0
Business days after the invoice date	16
Days after the due date	45
Wholesale accounts receivable written off	$ 0	0	$ 0
Margin deposit	8	33
Letters of Credit Outstanding, Amount	$ 17	$ 63
Public Utilities, Allowance for Funds Used During Construction, Rate	7.30%	7.30%	7.40%
Public Utilities, Allowance for Funds Used During Construction, Capitalized Interest	$ 11	$ 13	$ 22
Allowance for equity funds used during construction	21	21	$ 37
Construction in Progress, Gross	634
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	$ 213	$ 545
Depreciation expense rate	3.50%	3.60%	3.60%
Finite-Lived Intangible Assets, Accumulated Amortization	$ 257	$ 227
Amortization of Intangible Assets	44	$ 38	$ 25
Future Amortization Expense, Year One	45
Future Amortization Expense, Year Two	44
Future Amortization Expense, Year Three	38
Future Amortization Expense, Year Four	34
Future Amortization Expense, Year Five	22
Power Cost Deadband - Lower Threshold	15
Power Cost Deadband - Upper Threshold	$ 30
Public Utilities, Approved Return on Equity, Percentage	9.60%	9.68%	9.75%
Franchise taxes	$ 43	$ 43	$ 42
Deferred Tax Assets, Regulatory Assets and Liabilities	29	42
Public Utilities, Property, Plant and Equipment, Amount of Construction Work in Process Included in Rate Base				$ 514
excess tax benefits recognized related to share-based payment awards	1
Proceeds and Excess Tax Benefit from Share-based Compensation	3	2
Type of Arrangement [Member]
Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	514
Noncurrent Regulatory Assets [Member]
Property, Plant and Equipment [Line Items]
Deferred Tax Assets, Regulatory Assets and Liabilities	86	86
Noncurrent Regulatory Liabilities [Member]
Property, Plant and Equipment [Line Items]
Asset Retirement Obligations, Noncurrent	$ 49	$ 45