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Investments (Investments Under Equity Method of Accounting) (Parenthetical) (Detail) - USD ($) $ in Millions | Dec. 31, 2020 | Nov. 30, 2020 | Sep. 30, 2020 | Dec. 31, 2019
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Align RNG, LLC |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Unfunded Commitments | $ 59 |  |  |
Cove Point |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership percentage |  | 50.00% |  |
Atlantic Coast Pipeline |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Percentage ownership in total units | 53.00% |  |  | 48.00%
Liability associated with its investment | $ 1,100 |  |  |
Fowler Ridge |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Percentage of equity interest sold to noncontrolling interest owners |  |  | 50.00% |

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