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Income Taxes (Unrecognized Tax Benefits) (Parenthetical) (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Income Tax Contingency [Line Items] |  |  |  |
Acquired unrecognized tax benefits | $ 0 | $ 129 | [1] | $ 0
Increases-prior period positions | 18 | 0 |  | 10
Decreases in prior period position | $ 19 | 0 |  | $ 0
SCANA |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |
Acquired unrecognized tax benefits |  | 106 |  |
Increases-prior period positions |  | 76 |  |
Decreases in prior period position |  | $ 53 |  |
[1] | Acquired unrecognized tax benefits reflect $106 million plus increases in prior period positions of $76 million and decreases in prior period positions of $53 million that were recorded through purchase accounting.

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- Definition Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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