Derivatives - Schedule of Cumulative Basis Adjustments for Fair Value Hedges (Details) - Fair Value Hedging - Balance Sheet Location [Axis]: us-gaap:LongTermDebtAndCapitalLeaseObligations - USD ($) $ in Millions |
Jun. 30, 2026 |
Dec. 31, 2025 |
|---|---|---|
| Derivative [Line Items] | ||
| Carrying Amount of the Hedged Item | $ (3,687) | $ (3,742) |
| Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item | 169 | 156 |
| SOUTHERN Co GAS | ||
| Derivative [Line Items] | ||
| Carrying Amount of the Hedged Item | (442) | (446) |
| Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item | $ 56 | $ 51 |