REGULATORY MATTERS - GAS Schedule of Infrastructure Replacement Programs and Capital Projects (Details) - Southern Company Gas $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) mi	Dec. 31, 2022 USD ($)
Public Utilities, General Disclosures [Line Items]
Capital Expenditures in 2023	$ 553
Capital Expenditures Since Project Inception	$ 3,910
Pipe Installed Since Project Inception (in miles) | mi	1,969
Scope of Program (in miles) | mi	2,135
Investing In Illinois Qualifying Infrastructure Plant
Public Utilities, General Disclosures [Line Items]
Capital Expenditures in 2023	$ 365
Capital Expenditures Since Project Inception	$ 3,228
Pipe Installed Since Project Inception (in miles) | mi	1,367
Scope of Program (in miles) | mi	1,367
Program Duration	9 years
SAVE
Public Utilities, General Disclosures [Line Items]
Capital Expenditures in 2023	$ 75
Capital Expenditures Since Project Inception	$ 486
Pipe Installed Since Project Inception (in miles) | mi	567
Scope of Program (in miles) | mi	695
Program Duration	13 years
System Reinforcement Rider
Public Utilities, General Disclosures [Line Items]
Capital Expenditures in 2023	$ 104	$ 76
Capital Expenditures Since Project Inception	$ 180
Pipe Installed Since Project Inception (in miles) | mi	20
Program Duration	3 years
Pipeline Replacement Program
Public Utilities, General Disclosures [Line Items]
Capital Expenditures in 2023	$ 9
Capital Expenditures Since Project Inception	$ 16
Pipe Installed Since Project Inception (in miles) | mi	15
Scope of Program (in miles) | mi	73
Program Duration	7 years