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REGULATORY MATTERS - GAS Infrastructure Replacement Programs and Capital Projects and Natural Gas Recovery (Details) $ in Millions |  |  |  | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | ---
Dec. 24, 2024 USD ($) | Jul. 02, 2024 USD ($) | Jun. 07, 2024 USD ($) | Nov. 30, 2023 USD ($) | Dec. 31, 2023 USD ($) | Jun. 30, 2023 USD ($) | Dec. 31, 2024 USD ($) | Dec. 31, 2023 USD ($) | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($) mi | Dec. 31, 2019 USD ($) | Dec. 31, 2014
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  | $ (26,724.0) | $ (25,253.0) | $ (29,279.0) |  |  |
Total operating expenses |  |  |  |  |  |  | 19,656.0 | 19,427.0 | 23,909.0 |  |  |
Southern Company Gas |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Capital expenditures |  |  |  |  |  |  | 186.0 |  |  |  |  |
Pretax charge to income |  |  |  |  |  |  | 0.0 | 96.0 | 0.0 |  |  |
Revenues |  |  |  |  |  |  | (4,456.0) | (4,702.0) | (5,962.0) |  |  |
Total operating expenses |  |  |  |  |  |  | 3,329.0 | 3,763.0 | 5,148.0 |  |  |
Southern Company Gas | Natural Gas Cost Under Recovery |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Over (under) recovered fuel cost |  |  |  |  | $ 214.0 |  | (193.0) | 214.0 |  |  |  |
Southern Company Gas | SAVE |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Program duration period |  |  | 5 years |  |  |  |  |  |  |  |  |
Capital expenditures |  |  |  |  |  |  | 75.0 |  |  |  |  |
Infrastructure replacement program, approved investment variance amount |  |  | $ 5.0 |  |  |  |  |  |  |  |  |
Nicor Gas |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Infrastructure disallowed |  |  |  |  | 69.0 |  |  |  |  |  |  |
Pretax charge to income |  |  |  |  | 58.0 | $ 38.0 | (96.0) |  |  |  |  |
After tax charge to income |  |  |  |  | $ 44.0 | 28.0 | (72.0) |  |  |  |  |
Nicor Gas | Other Transmission and Distribution Capital Investments |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Infrastructure disallowed |  |  |  | $ 95.9 |  |  |  |  |  |  |  |
Nicor Gas | Investing In Illinois Qualifying Infrastructure Plant |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Infrastructure disallowed | $ 14.0 |  |  | 126.8 |  |  | $ 63.0 |  |  |  | $ 32.0 |
Capital expenditures |  |  |  |  |  |  |  |  |  |  | $ 415.0 |
Revenues |  |  |  |  |  | 8.0 |  |  |  |  |  |
Total operating expenses |  |  |  |  |  | $ 30.0 |  |  |  |  |  |
Nicor Gas | QIP Rider |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Infrastructure disallowed |  |  |  | $ 31.0 |  |  |  |  |  |  |  |
Nicor Gas | Maximum |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Infrastructure investment, annual customer rate increase |  |  |  |  |  |  |  |  |  |  |  | 4.00%
Infrastructure investment, annual customer rate increase in any given year |  |  |  |  |  |  |  |  |  |  |  | 5.50%
Virginia Natural Gas | SAVE |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Program duration period |  |  |  |  |  |  | 6 years |  |  |  |  |
Infrastructure replacement program, approved investment variance amount |  |  |  |  |  |  | $ 5.0 |  |  |  |  |
Atlanta Gas Light |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Gain contingency, unrecorded amount |  |  |  |  |  |  | 22.0 |  |  |  |  |
Annual proceeds from strategic economic development projects |  |  |  |  |  |  | 15.0 |  |  |  |  |
Approved annual recovery of capital investment |  | $ 600.0 |  |  |  |  |  |  | 286.0 |  |  |
Chattanooga Gas |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Program duration period |  |  |  |  |  |  |  |  |  | 7 years |  |
Pipeline replacement program, length of distribution main | mi |  |  |  |  |  |  |  |  |  | 73 |  |
Total project capital cost forecast |  |  |  |  |  |  |  |  |  | $ 118.0 |  |
Operating Segments |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  | (26,059.0) | (24,700.0) | (28,835.0) |  |  |
Operating Segments | Southern Company Gas |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  | (4,447.0) | (4,685.0) | (5,937.0) |  |  |
Operating Segments | Gas distribution operations | Southern Company Gas |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Gross property additions |  |  |  |  |  |  | 1,700.0 |  |  |  |  |
Revenues |  |  |  |  |  |  | $ (3,899.0) | $ (4,105.0) | $ (5,267.0) |  |  |

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