REGULATORY MATTERS - GAS Rate Proceedings and Deferral of Incremental COVID-19 Costs (Details) - USD ($) $ in Millions											1 Months Ended				3 Months Ended		12 Months Ended
	Jan. 01, 2027	Jan. 01, 2026	Jan. 03, 2025	Jan. 01, 2025	Dec. 24, 2024	Aug. 01, 2024	Jul. 02, 2024	Jun. 07, 2024	Jan. 01, 2024	Jan. 01, 2023	Nov. 30, 2023	Aug. 31, 2023	Nov. 30, 2021	Apr. 30, 2021	Dec. 31, 2023	Jun. 30, 2023	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2019
Public Utilities, General Disclosures [Line Items]
Net regulatory assets															$ 6,523.0		$ 7,041.0	$ 6,523.0
Nicor Gas
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount											$ 223.0
Authorized ROE											9.51%						9.51%
Public utilities, approved equity capital structure, percentage											50.00%
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 | Subsequent Event
Public Utilities, General Disclosures [Line Items]
Authorized ROE			10.35%
Public utilities, approved equity capital structure, percentage			54.60%
Requested rate increase (decrease), amount			$ 309.0
Nicor Gas | Investing In Illinois Qualifying Infrastructure Plant
Public Utilities, General Disclosures [Line Items]
Infrastructure disallowed					$ 14.0						$ 126.8						63.0			$ 32.0
Nicor Gas | QIP Rider
Public Utilities, General Disclosures [Line Items]
Infrastructure disallowed											31.0
Nicor Gas | Other Transmission and Distribution Capital Investments
Public Utilities, General Disclosures [Line Items]
Infrastructure disallowed											$ 95.9
Atlanta Gas Light
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount																	$ (9.0)	(7.0)	$ (5.0)
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													$ 1,700.0
Approved annual recovery of capital investment							$ 600.0												286.0
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														$ 500.0
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														$ 600.0
Southern Company Gas
Public Utilities, General Disclosures [Line Items]
Pretax charge to income																	$ 0.0	96.0	$ 0.0
Requested rate increase (decrease), amount									$ 53.0	$ 53.0
Net regulatory assets															$ (2,225.0)		$ (2,252.0)	$ (2,225.0)
Southern Company Gas | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amount	$ 74.0	$ 73.0		$ 72.0
Southern Company Gas | SAVE
Public Utilities, General Disclosures [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year								$ 355.0
Infrastructure replacement program, approved investment variance amount								$ 5.0
Program duration period								5 years
Southern Company Gas | SAVE | 2029
Public Utilities, General Disclosures [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year								$ 70.0
Southern Company Gas | SAVE | 2027
Public Utilities, General Disclosures [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year								70.0
Southern Company Gas | SAVE | 2025
Public Utilities, General Disclosures [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year								70.0
Southern Company Gas | SAVE | 2028
Public Utilities, General Disclosures [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year								70.0
Southern Company Gas | SAVE | 2026
Public Utilities, General Disclosures [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year								$ 70.0
Virginia Natural Gas
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount												$ 48.0
Authorized ROE												9.70%					9.70%
Public utilities, approved equity capital structure, percentage												49.06%
Requested rate increase (decrease), amount										$ 69.0
Virginia Natural Gas | Virginia Commission
Public Utilities, General Disclosures [Line Items]
Authorized ROE						10.45%
Public utilities, approved equity capital structure, percentage						54.92%
Requested rate increase (decrease), amount						$ 63.0
Virginia Natural Gas | SAVE
Public Utilities, General Disclosures [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year																	$ 365.0
Infrastructure replacement program, approved investment variance amount																	$ 5.0
Program duration period																	6 years
Virginia Natural Gas | SAVE | Virginia Commission
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amount						$ 17.0
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 2022
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