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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)