Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Property, Plant and Equipment [Line Items]
Other Investments	$ 0	$ 0
Margin deposit	46	80
Letters of Credit Outstanding, Amount	45	104
Public Utilities, Allowance for Funds Used During Construction, Rate	7.50%	7.80%	7.60%
Public Utilities, Allowance for Funds Used During Construction, Capitalized Interest	4	3	9
Allowance for equity funds used during construction	6	5	13
Depreciation expense rate	3.80%	3.70%	3.90%
Asset Retirement Removal Costs Included in Depreciation Expense	55	49	47
Finite-Lived Intangible Assets, Accumulated Amortization	151	153
Amortization of Intangible Assets	22	19	17
Future Amortization Expense, Year One	20
Future Amortization Expense, Year Two	18
Future Amortization Expense, Year Three	16
Future Amortization Expense, Year Four	14
Future Amortization Expense, Year Five	11
Power Cost Deadband - Lower Threshold	15
Power Cost Deadband - Upper Threshold	30
Net Variable Power Cost Underrun	(17)	(34)	(12)
Estimated Refund Under Power Cost Adjustment Mechanism	0	(10)	0
Potential Refund Under the PCAM		17
Actual Refund Under Power Cost Adjustment Mechanism		(6)
Franchise taxes	42	41	39
Deferred Tax Assets, Regulatory Assets and Liabilities	20	22
Noncurrent Regulatory Assets [Member]
Property, Plant and Equipment [Line Items]
Deferred Tax Assets, Regulatory Assets and Liabilities	$ 80	$ 87