REGULATORY MATTERS - APC Rate ERC (Details) - Alabama Power $ in Millions			1 Months Ended				12 Months Ended
	Dec. 01, 2020 USD ($) $ / KWH_Kilowatt_hour	Apr. 30, 2018 $ / KWH_Kilowatt_hour	Jan. 31, 2022 $ / KWH_Kilowatt_hour	Oct. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) $ / KWH_Kilowatt_hour	May 31, 2018 $ / KWH_Kilowatt_hour	Dec. 31, 2020 USD ($) $ / KWH_Kilowatt_hour	Aug. 07, 2020 USD ($)	Feb. 28, 2019 USD ($)	Dec. 31, 2018 USD ($)
Schedule of Regulatory Assets and Liabilities [Line Items]
Current billing rates under rate ECR (usd per KWH) | $ / KWH_Kilowatt_hour	1.960	0.02015			0.02160	0.02353	0.05910
Rate RSE refund liability					$ 53.0		$ 50.0		$ 78.0
Current billing rates under rate ECR, percentage decrease	1.84%				1.82%
Current billing rates under rate ECR, decrease, amount	$ 103.0				$ 102.0
Reduction of over-collected fuel balance				$ 94.3
Contract with customer, liability, bill credits					50.0			$ 100.0		$ 267.0
Over (under) recovered fuel cost					49.0		$ 18.0
Other regulatory liabilities, current
Schedule of Regulatory Assets and Liabilities [Line Items]
Over (under) recovered fuel cost					32.0
Other regulatory liabilities, deferred
Schedule of Regulatory Assets and Liabilities [Line Items]
Over (under) recovered fuel cost					$ 17.0
Scenario, Forecast
Schedule of Regulatory Assets and Liabilities [Line Items]
Current billing rates under rate ECR (usd per KWH) | $ / KWH_Kilowatt_hour			0.05910