Contingencies - Other Matters (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Loss Contingencies [Line Items]
Equity investments in unconsolidated subsidiaries	$ 1,368	$ 1,362
SOUTHERN Co GAS
Loss Contingencies [Line Items]
Equity investments in unconsolidated subsidiaries	1,290	1,290
SOUTHERN Co GAS \| PennEast Pipeline
Loss Contingencies [Line Items]
Construction contingency estimate	300
Equity investments in unconsolidated subsidiaries	93	91
SOUTHERN Co GAS \| SNG
Loss Contingencies [Line Items]
Equity investments in unconsolidated subsidiaries	$ 1,164	$ 1,167
Ownership percentage, equity method investment	50.00%
Commitment to fund contingent capital contribution	$ 150
SNG
Loss Contingencies [Line Items]
Debt maturing in June 2021	$ 300

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