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Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Property, Plant and Equipment [Line Items] |  |  |
Excise Taxes Collected | $ 45 | $ 45 | $ 43
Regulatory Liabilities | 1,421 | 1,391 |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 438 |  |
Other Investments | $ 26 | 112 |
Document Period End Date | Dec. 31, 2019 |  |
Days after the due date | 45 days |  |
Wholesale accounts receivable written off | $ 0 | 0 | $ 0
Margin deposit | 16 | 16 |
Letters of Credit Outstanding, Amount | $ 15 | 48 |
Public Utilities, Allowance for Funds Used During Construction, Rate | 7.10% |  | 7.30%
Public Utilities, Allowance for Funds Used During Construction, Capitalized Interest | $ 5 | 6 | $ 6
Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity | $ 10 | $ 11 | $ 12
Depreciation expense rate | 3.60% | 3.60% | 3.60%
Finite-Lived Intangible Assets, Accumulated Amortization | $ 366 | $ 302 |
Amortization of Intangible Assets | 64 | $ 59 | $ 46
Future Amortization Expense, Year One | 60 |  |
Future Amortization Expense, Year Two | 52 |  |
Future Amortization Expense, Year Three | 46 |  |
Future Amortization Expense, Year Four | 37 |  |
Future Amortization Expense, Year Five | 32 |  |
Power Cost Deadband - Lower Threshold | 15 |  |
Power Cost Deadband - Upper Threshold | $ 30 |  |
Public Utilities, Approved Return on Equity, Percentage | 9.50% | 9.50% | 9.60%
Quantifying Misstatement in Current Year Financial Statements, Amount | $ 3,917 | $ 3,838 |
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation | 4,095 | 3,803 |
Trojan decommissioning [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 91 | 25 |
Asset Retirement Obligation Costs [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Regulatory Liabilities | 54 | 53 |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 0 |  |
Deferred Income Tax Charge [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Regulatory Liabilities | 260 | $ 267 |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 0 |  |
Previously Reported [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Quantifying Misstatement in Current Year Financial Statements, Amount | 223 |  |
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation | 113 |  |
Accounting Standards Update 2016-02 [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | 44 |  |
Property, Plant and Equipment [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | (178) |  |
Property, Plant and Equipment [Member] | Building [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | 131 |  |
Property, Plant and Equipment [Member] | Assets Held under Capital Leases [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | 49 |  |
Property, Plant and Equipment [Member] | Accounting Standards Update 2016-02 [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | 2 |  |
AOCI Attributable to Parent [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Reclassification of stranded tax effects due to Tax Reform | $ 2 |  |

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