REGULATORY MATTERS - GAS Schedule of Infrastructure Replacement Programs and Capital Projects (Details) - Southern Company Gas $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) mi
Public Utilities, General Disclosures [Line Items]
Expenditures in 2020 | $	$ 438
Expenditures Since Project Inception | $	$ 2,394
Pipe Installed Since Project Inception (in miles) | mi	1,409
Scope of Program (in miles) | mi	2,220
Investing In Illinois
Public Utilities, General Disclosures [Line Items]
Expenditures in 2020 | $	$ 389
Expenditures Since Project Inception | $	$ 2,101
Pipe Installed Since Project Inception (in miles) | mi	996
Scope of Program (in miles) | mi	1,450
Program Duration	9 years
SAVE
Public Utilities, General Disclosures [Line Items]
Expenditures in 2020 | $	$ 49
Expenditures Since Project Inception | $	$ 293
Pipe Installed Since Project Inception (in miles) | mi	413
Scope of Program (in miles) | mi	770
Program Duration	13 years