Regulatory Matters - GAS Schedule of Infrastructure Replacement Programs and Capital Projects (Details) - SOUTHERN Co GAS $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($)
Public Utilities, General Disclosures [Line Items]
Capital expenditures	$ 343
Investing in Illinois
Public Utilities, General Disclosures [Line Items]
Capital expenditures	307
Steps to Advance Virginia's Energy
Public Utilities, General Disclosures [Line Items]
Capital expenditures	$ 36