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Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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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 |

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