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REGULATORY MATTERS - GAS Rate Proceedings and Deferral of Incremental COVID-19 Costs (Details) - USD ($) $ in Millions |  |  |  |  |  |  |  | 1 Months Ended | 12 Months Ended |  |
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Feb. 01, 2024 | Jan. 01, 2024 | Nov. 16, 2023 | Aug. 28, 2023 | Jan. 01, 2023 | Jan. 01, 2022 | Nov. 01, 2020 | Nov. 30, 2021 | Apr. 30, 2021 | Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Feb. 09, 2024 | Dec. 31, 2019
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net regulatory assets |  |  |  |  |  |  |  |  |  |  | $ 6,523 | $ 5,833 |  |  |
Nicor Gas |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved rate increase (decrease), amount |  |  | $ 223 |  |  |  |  | $ 240 |  |  |  |  |  |  |
Authorized ROE |  |  | 9.51% |  |  |  |  | 9.75% |  |  | 9.51% |  |  |  |
Public utilities, approved equity capital structure, percentage |  |  | 50.00% |  |  |  |  | 54.50% |  |  |  |  |  |  |
Nicor Gas | Investing In Illinois Qualifying Infrastructure Plant |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved rate increase amount recovery of investments |  |  |  |  |  |  |  | $ 94 |  |  |  |  |  |  |
Atlanta Gas Light |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved rate increase (decrease), amount |  |  |  |  |  | $ (43) |  |  |  |  | $ (7) | (5) |  |  |
Authorized ROE |  |  |  |  |  |  |  |  |  |  | 10.25% |  |  |  |
Public utilities, maximum base rate percentage |  |  |  |  |  |  |  |  |  |  | 5.00% |  |  |  |
Integrated capacity delivery plan, capital budgets and related operations and maintenance spending, term |  |  |  |  |  |  |  |  | 10 years |  |  |  |  |  |
Approved rate decrease, percent |  |  |  |  |  |  |  | 10.00% |  |  |  |  |  |  |
Integrated capacity delivery plan, capital forecast plan, years one through three |  |  |  |  |  |  |  | $ 1,700 |  |  |  |  |  |  |
Approved annual recovery of capital investment |  |  |  |  |  |  |  |  |  |  |  |  | $ 286 |  |
Atlanta Gas Light | Scenario, Forecast |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved rate increase (decrease), amount |  |  |  |  |  |  |  |  |  | $ (9) |  |  |  |  |
Atlanta Gas Light | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized ROE |  |  |  |  |  |  |  |  |  |  | 10.05% |  |  |  |
Integrated capacity delivery plan, capital forecast plan, years one through ten |  |  |  |  |  |  |  |  | $ 500 |  |  |  |  |  |
Atlanta Gas Light | Minimum | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved annual recovery of capital investment | $ 700 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Atlanta Gas Light | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized ROE |  |  |  |  |  |  |  |  |  |  | 10.45% |  |  |  |
Integrated capacity delivery plan, capital forecast plan, years one through ten |  |  |  |  |  |  |  |  | $ 600 |  |  |  |  |  |
Atlanta Gas Light | Maximum | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved annual recovery of capital investment | $ 1,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Virginia Natural Gas |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved rate increase (decrease), amount |  |  |  | $ 48 |  |  |  |  |  |  |  |  | 43 |  |
Public utilities, approved rate increase amount recovery of investments |  |  |  |  |  |  |  |  |  |  |  |  | $ 14 |  |
Authorized ROE |  |  |  | 9.70% |  |  |  |  |  |  | 9.70% |  | 9.50% |  |
Public utilities, approved equity capital structure, percentage |  |  |  | 49.06% |  |  |  |  |  |  |  |  | 51.90% |  |
Requested rate increase (decrease), amount |  |  |  |  | $ 69 |  | $ 50 |  |  |  |  |  |  |  |
Virginia Natural Gas | SAVE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved infrastructure replacement program, approved investment amount, current fiscal year |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 365
Southern Company Gas |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease), amount |  |  |  |  | $ 53 |  |  |  |  |  |  |  |  |  |
Net regulatory assets |  |  |  |  |  |  |  |  |  |  | $ (2,225) | $ (1,891) |  |  |
Southern Company Gas | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease), amount |  | $ 53 |  |  |  |  |  |  |  |  |  |  |  |  |
Southern Company Gas | SAVE | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved infrastructure replacement program, approved investment amount, current fiscal year |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 355 |
Southern Company Gas | SAVE | Subsequent Event | 2025 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved infrastructure replacement program, approved investment amount, current fiscal year |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 70 |

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