REGULATORY MATTERS - GAS Schedule of Infrastructure Replacement Programs and Capital Projects (Details) - Southern Company Gas $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) mi	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Public Utilities, General Disclosures [Line Items]
Expenditures in 2021	$ 461
Expenditures Since Project Inception	$ 2,852
Pipe Installed Since Project Inception (in miles) | mi	1,628
Scope of Program (in miles) | mi	2,107
Investing In Illinois
Public Utilities, General Disclosures [Line Items]
Expenditures in 2021	$ 408
Expenditures Since Project Inception	$ 2,508
Pipe Installed Since Project Inception (in miles) | mi	1,153
Scope of Program (in miles) | mi	1,394
Program Duration	9 years
Steps to Advance Virginia's Energy (SAVE)
Public Utilities, General Disclosures [Line Items]
Expenditures in 2021	$ 51	$ 49	$ 45
Expenditures Since Project Inception	$ 342
Pipe Installed Since Project Inception (in miles) | mi	470
Scope of Program (in miles) | mi	640
Program Duration	13 years
System Reinforcement Rider
Public Utilities, General Disclosures [Line Items]
Expenditures in 2021	$ 0
Expenditures Since Project Inception	$ 0
Program Duration	3 years
Pipeline Replacement Program
Public Utilities, General Disclosures [Line Items]
Expenditures in 2021	$ 2
Expenditures Since Project Inception	$ 2
Pipe Installed Since Project Inception (in miles) | mi	5
Scope of Program (in miles) | mi	73
Program Duration	7 years