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Income Taxes (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2020 | Dec. 31, 2018 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Income Tax Disclosure [Line Items] |  |  |  |  |
Refund of AMT credit carryforwards |  |  | $ 572 | $ 573 |
Deferred tax liabilities |  |  | $ 117 |  |
Reclassification of AMT credits from noncurrent deferred tax liabilities to a current federal income tax receivable |  |  |  | 286 |
AMT credit carryforwards |  |  |  |  |
Income Tax Disclosure [Line Items] |  |  |  |  |
Refund of AMT credit carryforwards | $ 572 |  |  | $ 573 |
Income tax interest income | $ 19 |  |  |  |
Analysis of existing regulatory liability |  |  |  |  |
Income Tax Disclosure [Line Items] |  |  |  |  |
Measurement period adjustment |  |  |  |  | $ 23
Bonus depreciation |  |  |  |  |
Income Tax Disclosure [Line Items] |  |  |  |  |
Measurement period adjustment |  |  |  |  | 10
Nondeductible long-term incentives |  |  |  |  |
Income Tax Disclosure [Line Items] |  |  |  |  |
Measurement period adjustment |  |  |  |  | 7
Refundable AMT credits |  |  |  |  |
Income Tax Disclosure [Line Items] |  |  |  |  |
Measurement period adjustment |  | $ 76 |  |  |
Regulatory liability |  |  |  |  |
Income Tax Disclosure [Line Items] |  |  |  |  |
Measurement period adjustment |  |  |  |  | 83
Reclassified AMT credit carryforwards |  |  |  |  |
Income Tax Disclosure [Line Items] |  |  |  |  |
Measurement period adjustment |  |  |  |  | $ 573

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- Definition Amount, before allocation of valuation allowances, of deferred tax asset attributable to deductible differences from reserves and accruals, compensation and benefit costs, and other provisions, reserves, and allowances. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32632-109319 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32621-109319 + Details
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