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Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Property, Plant and Equipment [Line Items] |  |  |
Excise Taxes Collected | $ 46 | $ 45 | $ 45
Regulatory Liabilities | 1,392 | 1,421 |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 506 |  |
Other Investments | $ 255 | 26 |
Document Period End Date | Dec. 31, 2020 |  |
Days after the due date | 45 days |  |
Wholesale accounts receivable written off | $ 0 | 0 | $ 0
Margin deposit | 8 | 16 |
Letters of Credit Outstanding, Amount | $ 12 | $ 15 |
Public Utilities, Allowance for Funds Used During Construction, Rate | 6.90% | 7.10% | 7.30%
Public Utilities, Allowance for Funds Used During Construction, Capitalized Interest | $ 8 | $ 5 | $ 6
Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity | $ 16 | $ 10 | $ 11
Depreciation expense rate | 3.50% | 3.60% | 3.60%
Finite-Lived Intangible Assets, Accumulated Amortization | $ 388 | $ 366 |
Amortization of Intangible Assets | 64 | $ 64 | $ 59
Future Amortization Expense, Year One | 57 |  |
Future Amortization Expense, Year Two | 51 |  |
Future Amortization Expense, Year Three | 42 |  |
Future Amortization Expense, Year Four | 37 |  |
Future Amortization Expense, Year Five | 25 |  |
Power Cost Deadband - Lower Threshold | 15 |  |
Power Cost Deadband - Upper Threshold | $ 30 |  |
Public Utilities, Approved Return on Equity, Percentage | 9.50% | 9.50% |
Quantifying Misstatement in Current Year Financial Statements, Amount | $ 4,136 | $ 3,917 |
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation | 3,864 | 4,095 |
Asset Retirement Obligation, Cash Paid to Settle | 18 | 9 | $ 5
Deferred Purchased Power Costs | 114 |  |
Unbilled Receivables, Current | 97 | 86 |
net variable power cost amount below the baseline | 13 |  |
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 | 88 | 91 |
Asset Retirement Obligation Costs [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Regulatory Liabilities | 37 | 54 |
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 | 239 | 260 |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | $ 0 |  |
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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