REGULATORY MATTERS - GAS Schedule of Infrastructure Replacement Programs and Capital Projects (Details) - Southern Company Gas $ in Millions	12 Months Ended
	Dec. 31, 2022 USD ($) mi	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Public Utilities, General Disclosures [Line Items]
Expenditures in 2022	$ 587
Expenditures Since Project Inception	$ 3,439
Pipe Installed Since Project Inception (in miles) | mi	1,837
Scope of Program (in miles) | mi	2,622
Investing In Illinois
Public Utilities, General Disclosures [Line Items]
Expenditures in 2022	$ 437
Expenditures Since Project Inception	$ 2,945
Pipe Installed Since Project Inception (in miles) | mi	1,297
Scope of Program (in miles) | mi	1,854
Program Duration	9 years
Steps to Advance Virginia's Energy (SAVE)
Public Utilities, General Disclosures [Line Items]
Expenditures in 2022	$ 69	$ 51	$ 49
Expenditures Since Project Inception	$ 411
Pipe Installed Since Project Inception (in miles) | mi	525
Scope of Program (in miles) | mi	695
Program Duration	13 years
System Reinforcement Rider
Public Utilities, General Disclosures [Line Items]
Expenditures in 2022	$ 76
Expenditures Since Project Inception	$ 76
Pipe Installed Since Project Inception (in miles) | mi	10
Program Duration	3 years
Pipeline Replacement Program
Public Utilities, General Disclosures [Line Items]
Expenditures in 2022	$ 5
Expenditures Since Project Inception	$ 7
Pipe Installed Since Project Inception (in miles) | mi	5
Scope of Program (in miles) | mi	73
Program Duration	7 years