Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Property, Plant and Equipment [Line Items]
Other Investments						$ 0
Margin deposit						80	83
Letters of Credit Outstanding, Amount						104	180
Public Utilities, Allowance for Funds Used During Construction, Rate						8.00%	8.00%	7.00%
Public Utilities, Allowance for Funds Used During Construction, Capitalized Interest						2.7	9.0	12.0
Allowance for equity funds used during construction						5	13	18
Depreciation expense rate						3.71%	3.90%	3.80%
asset retirement removal costs included in depreciation expense						49	47	47
Finite-Lived Intangible Assets, Accumulated Amortization						153	133
Amortization of Intangible Assets						19	17	16
Future Amortization Expense, Year One					20
Future Amortization Expense, Year Two				14
Future Amortization Expense, Year Three			12
Future Amortization Expense, Year Four		11
Future Amortization Expense, Year Five	8
power cost deadband - lower threshold						15	17	15
power cost deadband - upper threshold						30	35	29
net variable power cost underrun						(34)	(12)
net variable power cost savings outside of threshold						19
Estimated Refund Under Power Cost Adjustment Mechanism						(10)	0	0
potential refund under the PCAM						17
net variable power cost overrun							22
Franchise taxes						41	39	38
Deferred Tax Assets, Regulatory Assets and Liabilities						22	37
Noncurrent Regulatory Assets [Member]
Property, Plant and Equipment [Line Items]
Deferred Tax Assets, Regulatory Assets and Liabilities						$ 87	$ 95