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Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Property, Plant and Equipment [Line Items] |  |  |
Other Investments | $ 30 | $ 1 |
Business days after the invoice date | 16 |  |
Days after the due date | 45 |  |
Wholesale accounts receivable written off | $ 0 | 0 | $ 0
Margin deposit | 11 | 8 |
Letters of Credit Outstanding, Amount | $ 31 | $ 17 |
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 | $ 11 | $ 13
Allowance for equity funds used during construction | $ 12 | $ 21 | $ 21
Depreciation expense rate | 3.60% | 3.50% | 3.60%
Finite-Lived Intangible Assets, Accumulated Amortization | $ 296 | $ 257 |
Amortization of Intangible Assets | 46 | $ 44 | $ 38
Future Amortization Expense, Year One | 49 |  |
Future Amortization Expense, Year Two | 48 |  |
Future Amortization Expense, Year Three | 43 |  |
Future Amortization Expense, Year Four | 35 |  |
Future Amortization Expense, Year Five | 28 |  |
Power Cost Deadband - Lower Threshold | 15 |  |
Power Cost Deadband - Upper Threshold | $ 30 |  |
Public Utilities, Approved Return on Equity, Percentage | 9.60% | 9.60% | 9.68%
Regulatory Liability, Noncurrent | $ 1,288 | $ 958 |
Franchise taxes | 43 | 43 | $ 43
Regulatory Assets, Noncurrent | 438 | 498 |
Defined Benefit Plan, Interest Cost | 3 |  |
Deferred Income Tax Charge [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Regulatory Assets, Noncurrent | 0 | 86 |
Asset Retirement Obligation Costs [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Regulatory Liability, Noncurrent | 52 | 49 |
Deferred Income Tax Charge [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Regulatory Liability, Noncurrent | $ 277 | $ 0 |

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