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Fair Value Measurements (Tables) | 9 Months Ended
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Sep. 30, 2025
Fair Value Disclosures [Abstract] |
Schedule of Assets and Liabilities Measured at Fair Value | At September 30, 2025, assets and liabilities measured at fair value on a recurring basis during the period, together with their associated level of the fair value hierarchy, were as follows:
| Fair Value Measurements Using |  |
At September 30, 2025 | Quoted Prices in Active Markets for Identical Assets (Level 1) |  | Significant Other Observable Inputs (Level 2) |  | Significant Unobservable Inputs (Level 3) |  | Net Asset Value as a Practical Expedient (NAV) |  | Total
| (in millions)
Southern Company |  |  |  |  |  |  |  |  |
Assets: |  |  |  |  |  |  |  |  |
Energy-related derivatives(a) | $ | 15 |  |  | $ | 89 |  |  | $ | — |  |  | $ | — |  |  | $ | 104 |
Interest rate derivatives | — |  |  | 7 |  |  | — |  |  | — |  |  | 7 |
Foreign currency derivatives | — |  |  | 31 |  |  | — |  |  | — |  |  | 31 |
Investments in trusts:(b) |  |  |  |  |  |  |  |  |
Domestic equity | 934 |  |  | 288 |  |  | — |  |  | — |  |  | 1,222 |
Foreign equity | 174 |  |  | 216 |  |  | — |  |  | — |  |  | 390 |
U.S. Treasury and government agency securities | — |  |  | 367 |  |  | — |  |  | — |  |  | 367 |
Municipal bonds | — |  |  | 50 |  |  | — |  |  | — |  |  | 50 |
Pooled funds – fixed income | — |  |  | 6 |  |  | — |  |  | — |  |  | 6 |
Corporate bonds | — |  |  | 505 |  |  | — |  |  | — |  |  | 505 |
Mortgage- and asset-backed securities | — |  |  | 120 |  |  | — |  |  | — |  |  | 120 |
Private equity | — |  |  | — |  |  | — |  |  | 186 |  |  | 186 |
Cash and cash equivalents | 1 |  |  | — |  |  | — |  |  | — |  |  | 1 |
Other | 40 |  |  | 4 |  |  | — |  |  | 9 |  |  | 53 |
Investments, available-for-sale: |  |  |  |  |  |  |  |  |
U.S. Treasury and government agency securities | 3 |  |  | 10 |  |  | — |  |  | — |  |  | 13 |
Corporate bonds | — |  |  | 2 |  |  | — |  |  | — |  |  | 2 |
Mortgage- and asset-backed securities | — |  |  | 7 |  |  | — |  |  | — |  |  | 7 |
Cash equivalents and restricted cash | 2,547 |  |  | 17 |  |  | — |  |  | — |  |  | 2,564 |
Other investments | 10 |  |  | 36 |  |  | 8 |  |  | — |  |  | 54 |
Total | $ | 3,724 |  |  | $ | 1,755 |  |  | $ | 8 |  |  | $ | 195 |  |  | $ | 5,682 |
Liabilities: |  |  |  |  |  |  |  |  |
Energy-related derivatives(a) | $ | 12 |  |  | $ | 77 |  |  | $ | — |  |  | $ | — |  |  | $ | 89 |
Interest rate derivatives | — |  |  | 191 |  |  | — |  |  | — |  |  | 191 |
Foreign currency derivatives | — |  |  | 21 |  |  | — |  |  | — |  |  | 21 |
Contingent consideration | 3 |  |  | — |  |  | 14 |  |  | — |  |  | 17 |
Other | — |  |  | 13 |  |  | 11 |  |  | — |  |  | 24 |
Total | $ | 15 |  |  | $ | 302 |  |  | $ | 25 |  |  | $ | — |  |  | $ | 342 |
| Fair Value Measurements Using |  |
At September 30, 2025 | Quoted Prices in Active Markets for Identical Assets (Level 1) |  | Significant Other Observable Inputs (Level 2) |  | Significant Unobservable Inputs (Level 3) |  | Net Asset Value as a Practical Expedient (NAV) |  | Total
| (in millions)
Alabama Power |  |  |  |  |  |  |  |  |
Assets: |  |  |  |  |  |  |  |  |
Energy-related derivatives | $ | — |  |  | $ | 31 |  |  | $ | — |  |  | $ | — |  |  | $ | 31 |
Nuclear decommissioning trusts:(b) |  |  |  |  |  |  |  |  |
Domestic equity | 499 |  |  | 278 |  |  | — |  |  | — |  |  | 777 |
Foreign equity | 174 |  |  | — |  |  | — |  |  | — |  |  | 174 |
U.S. Treasury and government agency securities | — |  |  | 16 |  |  | — |  |  | — |  |  | 16 |
Municipal bonds | — |  |  | 1 |  |  | — |  |  | — |  |  | 1 |
Corporate bonds | — |  |  | 307 |  |  | — |  |  | — |  |  | 307 |
Mortgage- and asset-backed securities | — |  |  | 33 |  |  | — |  |  | — |  |  | 33 |
Private equity | — |  |  | — |  |  | — |  |  | 186 |  |  | 186 |
Other | 19 |  |  | 2 |  |  | — |  |  | 9 |  |  | 30 |
Cash equivalents | 264 |  |  | 17 |  |  | — |  |  | — |  |  | 281 |
Other investments | — |  |  | 36 |  |  | — |  |  | — |  |  | 36 |
Total | $ | 956 |  |  | $ | 721 |  |  | $ | — |  |  | $ | 195 |  |  | $ | 1,872 |
Liabilities: |  |  |  |  |  |  |  |  |
Energy-related derivatives | $ | — |  |  | $ | 25 |  |  | $ | — |  |  | $ | — |  |  | $ | 25 |
Georgia Power |  |  |  |  |  |  |  |  |
Assets: |  |  |  |  |  |  |  |  |
Energy-related derivatives | $ | — |  |  | $ | 27 |  |  | $ | — |  |  | $ | — |  |  | $ | 27 |
Nuclear decommissioning trusts:(b) |  |  |  |  |  |  |  |  |
Domestic equity | 435 |  |  | 1 |  |  | — |  |  | — |  |  | 436 |
Foreign equity | — |  |  | 215 |  |  | — |  |  | — |  |  | 215 |
U.S. Treasury and government agency securities | — |  |  | 351 |  |  | — |  |  | — |  |  | 351 |
Municipal bonds | — |  |  | 49 |  |  | — |  |  | — |  |  | 49 |
Corporate bonds | — |  |  | 198 |  |  | — |  |  | — |  |  | 198 |
Mortgage- and asset-backed securities | — |  |  | 87 |  |  | — |  |  | — |  |  | 87 |
Other | 21 |  |  | 2 |  |  | — |  |  | — |  |  | 23 |
Cash equivalents | 844 |  |  | — |  |  | — |  |  | — |  |  | 844 |
Total | $ | 1,300 |  |  | $ | 930 |  |  | $ | — |  |  | $ | — |  |  | $ | 2,230 |
Liabilities: |  |  |  |  |  |  |  |  |
Energy-related derivatives | $ | — |  |  | $ | 24 |  |  | $ | — |  |  | $ | — |  |  | $ | 24 |
| Fair Value Measurements Using |  |
At September 30, 2025 | Quoted Prices in Active Markets for Identical Assets (Level 1) |  | Significant Other Observable Inputs (Level 2) |  | Significant Unobservable Inputs (Level 3) |  | Net Asset Value as a Practical Expedient (NAV) |  | Total
| (in millions)
Mississippi Power |  |  |  |  |  |  |  |  |
Assets: |  |  |  |  |  |  |  |  |
Energy-related derivatives | $ | — |  |  | $ | 20 |  |  | $ | — |  |  | $ | — |  |  | $ | 20 |
Cash equivalents | 13 |  |  | — |  |  | — |  |  | — |  |  | 13 |
Total | $ | 13 |  |  | $ | 20 |  |  | $ | — |  |  | $ | — |  |  | $ | 33 |
Liabilities: |  |  |  |  |  |  |  |  |
Energy-related derivatives | $ | — |  |  | $ | 23 |  |  | $ | — |  |  | $ | — |  |  | $ | 23 |
Southern Power |  |  |  |  |  |  |  |  |
Assets: |  |  |  |  |  |  |  |  |
Energy-related derivatives | $ | — |  |  | $ | 4 |  |  | $ | — |  |  | $ | — |  |  | $ | 4 |
Foreign currency derivatives | — |  |  | 20 |  |  | — |  |  | — |  |  | 20 |
Cash equivalents | 813 |  |  | — |  |  | — |  |  | — |  |  | 813 |
Total | $ | 813 |  |  | $ | 24 |  |  | $ | — |  |  | $ | — |  |  | $ | 837 |
Liabilities: |  |  |  |  |  |  |  |  |
Energy-related derivatives | $ | — |  |  | $ | 1 |  |  | $ | — |  |  | $ | — |  |  | $ | 1 |
Contingent consideration | 3 |  |  | — |  |  | 14 |  |  | — |  |  | 17 |
Other | — |  |  | 13 |  |  | 11 |  |  | — |  |  | 24 |
Total | $ | 3 |  |  | $ | 14 |  |  | $ | 25 |  |  | $ | — |  |  | $ | 42 |
Southern Company Gas |  |  |  |  |  |  |  |  |
Assets: |  |  |  |  |  |  |  |  |
Energy-related derivatives(a) | $ | 15 |  |  | $ | 7 |  |  | $ | — |  |  | $ | — |  |  | $ | 22 |
Non-qualified deferred compensation trusts: |  |  |  |  |  |  |  |  |
Domestic equity | — |  |  | 9 |  |  | — |  |  | — |  |  | 9 |
Foreign equity | — |  |  | 1 |  |  | — |  |  | — |  |  | 1 |
Pooled funds – fixed income | — |  |  | 6 |  |  | — |  |  | — |  |  | 6 |
Cash and cash equivalents | 1 |  |  | — |  |  | — |  |  | — |  |  | 1 |
Cash equivalents | 329 |  |  | — |  |  | — |  |  | — |  |  | 329 |
Total | $ | 345 |  |  | $ | 23 |  |  | $ | — |  |  | $ | — |  |  | $ | 368 |
Liabilities: |  |  |  |  |  |  |  |  |
Energy-related derivatives(a) | $ | 12 |  |  | $ | 4 |  |  | $ | — |  |  | $ | — |  |  | $ | 16 |
Interest rate derivatives | — |  |  | 56 |  |  | — |  |  | — |  |  | 56 |
Total | $ | 12 |  |  | $ | 60 |  |  | $ | — |  |  | $ | — |  |  | $ | 72 |
(a)Excludes cash collateral of $10 million. (b)Excludes receivables related to investment income, pending investment sales, payables related to pending investment purchases, and currencies. See Note 6 to the financial statements under "Nuclear Decommissioning" in Item 8 of the Form 10-K for additional information.
Schedule of Increase (Decrease) In Fair Value Of Funds | The changes were recorded as a change to the regulatory assets and liabilities related to AROs for Georgia Power and Alabama Power, respectively.
| Three Months Ended September 30, | Nine Months Ended September 30,
Fair value increases (decreases) | 2025 | 2024 | 2025 | 2024
| (in millions)
Southern Company | $ | 132 |  | $ | 97 |  | $ | 290 |  | $ | 230 |
Alabama Power | 75 |  | 67 |  | 160 |  | 153 |
Georgia Power | 57 |  | 30 |  | 130 |  | 77 |
Schedule of Financial Instruments for which Carrying Amount Did Not Equal Fair Value | At September 30, 2025, other financial instruments for which the carrying amount did not equal fair value were as follows:
| Southern Company(*) | Alabama Power | Georgia Power | Mississippi Power | Southern Power | Southern Company Gas(*)
| (in billions)
Long-term debt, including securities due within one year: |  |  |
Carrying amount | $ | 71.9 |  | $ | 12.0 |  | $ | 20.8 |  | $ | 1.8 |  | $ | 3.8 |  | $ | 9.3 |
Fair value | 68.4 |  | 10.9 |  | 19.5 |  | 1.6 |  | 3.8 |  | 8.5 |
(*)The carrying amount of Southern Company Gas' long-term debt includes fair value adjustments from the effective date of the 2016 merger with Southern Company. Southern Company Gas amortizes the fair value adjustments over the remaining lives of the respective bonds, the latest being through 2043.

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- Definition Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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