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REGULATORY MATTERS - GAS Infrastructure Replacement Programs and Capital Projects (Details) $ in Millions |  |  |  | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | ---
Feb. 09, 2024 USD ($) | Feb. 01, 2024 USD ($) | Nov. 16, 2023 USD ($) | Dec. 31, 2023 USD ($) | Dec. 31, 2023 USD ($) | Dec. 31, 2021 USD ($) mi | Dec. 31, 2019 USD ($) | Dec. 31, 2014 | Dec. 31, 2022 USD ($) | Dec. 31, 2020 USD ($)
Southern Company Gas |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Capital expenditures |  |  |  |  | $ 553.0 |  |  |  |  |
Southern Company Gas | Natural Gas Cost Under Recovery |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Over (under) recovered fuel cost |  |  |  | $ (214.0) | (214.0) |  |  |  | $ 108.0 |
Southern Company Gas | SAVE |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Capital expenditures |  |  |  |  | 75.0 |  |  |  |  |
Southern Company Gas | SAVE | Subsequent Event |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Approved infrastructure replacement program, approved investment amount, current fiscal year | $ 355.0 |  |  |  |  |  |  |  |  |
Program duration period | 5 years |  |  |  |  |  |  |  |  |
Infrastructure replacement program, approved investment variance amount | $ 5.0 |  |  |  |  |  |  |  |  |
Southern Company Gas | SAVE | 2025 | Subsequent Event |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Approved infrastructure replacement program, approved investment amount, current fiscal year | 70.0 |  |  |  |  |  |  |  |  |
Southern Company Gas | SAVE | 2026 | Subsequent Event |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Approved infrastructure replacement program, approved investment amount, current fiscal year | 70.0 |  |  |  |  |  |  |  |  |
Southern Company Gas | SAVE | 2027 | Subsequent Event |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Approved infrastructure replacement program, approved investment amount, current fiscal year | 70.0 |  |  |  |  |  |  |  |  |
Southern Company Gas | SAVE | 2028 | Subsequent Event |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Approved infrastructure replacement program, approved investment amount, current fiscal year | 70.0 |  |  |  |  |  |  |  |  |
Southern Company Gas | SAVE | 2029 | Subsequent Event |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Approved infrastructure replacement program, approved investment amount, current fiscal year | $ 70.0 |  |  |  |  |  |  |  |  |
Southern Company Gas | Investing In Illinois Qualifying Infrastructure Plant |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Capital expenditures |  |  |  |  | 365.0 |  |  |  |  |
Nicor Gas |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Infrastructure disallowed |  |  |  | 69.0 |  |  |  |  |  |
Nicor Gas | Regulatory Infrastructure Program |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Program duration period |  |  |  |  |  |  |  | 9 years |  |
Nicor Gas | Investing In Illinois Qualifying Infrastructure Plant |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Infrastructure disallowed |  |  | $ 126.8 |  | 63.0 |  | $ 32.0 |  |  |
Capital expenditures |  |  |  |  |  |  | 415.0 |  |  |
Nicor Gas | Other Transmission and Distribution Capital Investments |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Infrastructure disallowed |  |  | $ 95.9 |  |  |  |  |  |  |
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] |  |  |  |  |  |  |  |  |  |
Approved infrastructure replacement program, approved investment amount, current fiscal year |  |  |  |  |  |  | $ 365.0 |  |  |
Program duration period |  |  |  |  |  |  | 6 years |  |  |
Infrastructure replacement program, approved investment variance amount |  |  |  |  |  |  |  |  |  | $ 5.0
Virginia Natural Gas | SAVE | Year 2021 |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Approved infrastructure replacement program, approved investment amount, current fiscal year |  |  |  |  |  |  |  |  |  | 60.0
Virginia Natural Gas | SAVE | Year 2022 |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Approved infrastructure replacement program, approved investment amount, current fiscal year |  |  |  |  |  |  |  |  |  | 70.0
Virginia Natural Gas | SAVE | Year 2023 |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Approved infrastructure replacement program, approved investment amount, current fiscal year |  |  |  |  |  |  |  |  |  | 70.0
Virginia Natural Gas | SAVE | Year 2024 |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Approved infrastructure replacement program, approved investment amount, current fiscal year |  |  |  |  |  |  |  |  |  | $ 70.0
Atlanta Gas Light |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Gain contingency, unrecorded amount |  |  |  | 44.0 | 44.0 |  |  |  |  |
Annual proceeds from strategic economic development projects |  |  |  |  | 15.0 |  |  |  |  |
Approved annual recovery of capital investment |  |  |  |  |  | $ 286.0 |  |  |  |
Atlanta Gas Light | True-Up Recovery, Unrecognized Equity |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Gain contingency, unrecorded amount |  |  |  | $ 23.0 | 23.0 |  |  |  |  |
Atlanta Gas Light | Maximum | Subsequent Event |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Approved annual recovery of capital investment |  | $ 1,000.0 |  |  |  |  |  |  |  |
Atlanta Gas Light | Minimum | Subsequent Event |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Approved annual recovery of capital investment |  | $ 700.0 |  |  |  |  |  |  |  |
Chattanooga Gas |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Program duration period |  |  |  |  |  | 7 years |  |  |  |
Pipeline replacement program, length of distribution main | mi |  |  |  |  |  | 73 |  |  |  |
Preliminary estimated cost to complete |  |  |  |  |  | $ 118.0 |  |  |  |
Operating Segments | Gas distribution operations | Southern Company Gas |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Gross property additions |  |  |  |  | $ 1,600.0 |  |  |  |  |

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