REGULATORY MATTERS - APC Regulatory Matters (Details)						1 Months Ended			12 Months Ended					25 Months Ended
	Jan. 01, 2021 USD ($)	Dec. 03, 2019 USD ($) $ / KWH_Kilowatt_hour	Dec. 02, 2019 $ / KWH_Kilowatt_hour	Oct. 01, 2019	May 01, 2018 USD ($)	Jan. 31, 2021 $ / KWH_Kilowatt_hour	Dec. 31, 2019 USD ($) MW	Jan. 31, 2019	Dec. 31, 2025	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) $ / KWH_Kilowatt_hour MW	Dec. 31, 2018 USD ($) $ / KWH_Kilowatt_hour	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($) MW	Mar. 31, 2020 USD ($)	Sep. 06, 2019 MW	May 08, 2019 MW	Apr. 15, 2019 USD ($)	Feb. 06, 2019 USD ($)	Feb. 05, 2019 USD ($)	Jan. 31, 2017 USD ($)
Schedule of Regulatory Assets and Liabilities [Line Items]
Contract with customer, liability, bill credits											$ 50,000,000
Other regulatory assets | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory amortization period											8 years
Alabama Power
Schedule of Regulatory Assets and Liabilities [Line Items]
Additional generating capacity, petitioned megawatts | MW																200
Additional generating capacity, petitioned megawatts, total | MW																2,400
Capital investment, construction and acquisition							$ 1,100,000,000				$ 1,100,000,000			$ 1,100,000,000
PPA, additional generating capacity (in MWs) | MW							640				640			640
Regulatory amortization period				5 years
Rate adjustment period											2 years
Maximum percentage of rate RSE											4.00%
Maximum annual percentage of ratio rate											5.00%
Adjusting point of weighted cost of equity							5.98%				5.98%			5.98%
Percent of basis points							0.07%				0.07%			0.07%
Percent of designated customer value benchmark survey							33.33%				33.33%			33.33%
Rate RSE refund liability													$ 53,000,000						$ 78,000,000	$ 78,000,000
Rate RSE increase																					4.48%
Rate RSE increase amount																					$ 245,000,000
Contract with customer, liability, bill credits											$ 267,000,000
Public utilities, approved return on equity											50.00%	47.00%
Weighted common equity return								6.15%				6.21%
Customer refundable fees, refund payments, tranche one, percentage											25.00%
Customer refundable fees, refund payments, tranche two, percentage											40.00%
Customer refundable fees, refund payments, tranche two, percentage											75.00%
Customer refundable fees, refund payments, tranche one, review year with rate RSE upward adjustment, percentage											50.00%
Under recovered certified power purchase agreements							$ 40,000,000				$ 40,000,000	$ 25,000,000		$ 40,000,000
Over (under) recovered environmental clause							62,000,000				62,000,000	$ (42,000,000)		62,000,000
Retail revenue requirement for environmental compliance											$ 205,000,000
Current billing rates under rate ECR (usd per KWH) | $ / KWH_Kilowatt_hour		0.02160	0.02353								0.05910	0.02015
Current billing rates under rate ECR, percentage decrease		1.82%
Current billing rates under rate ECR, decrease, amount		$ 102,000,000
Under recovered fuel cost							49,000,000				$ 49,000,000	$ 109,000,000		49,000,000
Tax Cuts and Jobs Act of 2017, change in tax rate, deferred tax liability, income tax benefit, maximum amount used to offset under recovered amounts					$ 30,000,000
Tax Cuts and Jobs Act of 2017, change in tax rate, deferred tax liability, income tax benefit											69,000,000
Amount offset under recovered balance											30,000,000
Tax Cuts and Jobs Act of 2017, change in tax rate, deferred tax liability, income tax benefit, amount increasing NDR balance		$ 39,000,000
Minimum natural disaster reserve balance, triggering establishment charge							50,000,000				50,000,000			50,000,000
Natural disaster reserve authorized limit							75,000,000				$ 75,000,000			75,000,000
Annual recovery amount														16,000,000
Maximum period for recovery deferred stock related operations and maintenance costs and any future reserve deficits											24 months
Maximum rate NDR charge per month, monthly nonresidential customer account							10				$ 10			10
Maximum rate NDR charge per month, monthly residential customer account							5				5			5
Alabama Power | Plant Barry Unit 8
Schedule of Regulatory Assets and Liabilities [Line Items]
Approximate nameplate capacity (in MW) | MW																	720
Costs incurred, percentage of estimated in-service cost				5.00%
Alabama Power | Scenario, Forecast
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved return on equity									55.00%
Retail revenue requirement for environmental compliance										$ 68,000,000
Current billing rates under rate ECR (usd per KWH) | $ / KWH_Kilowatt_hour						0.05910
Natural disaster reserve authorized limit	$ 50,000,000														$ 75,000,000
Annual recovery amount	$ 3,000,000									5,000,000
Alabama Power | Regulatory Liabilities, Current
Schedule of Regulatory Assets and Liabilities [Line Items]
Over (under) recovered environmental clause							(55,000,000)				(55,000,000)			(55,000,000)
Alabama Power | Regulatory Liabilities, Noncurrent
Schedule of Regulatory Assets and Liabilities [Line Items]
Over (under) recovered environmental clause							(7,000,000)				(7,000,000)			(7,000,000)
Alabama Power | Other regulatory liabilities, current
Schedule of Regulatory Assets and Liabilities [Line Items]
Under recovered fuel cost							32,000,000				32,000,000			32,000,000
Alabama Power | Other regulatory liabilities, deferred
Schedule of Regulatory Assets and Liabilities [Line Items]
Under recovered fuel cost							$ 17,000,000				$ 17,000,000			$ 17,000,000
Alabama Power | Minimum
Schedule of Regulatory Assets and Liabilities [Line Items]
Weighted cost of equity							5.75%				5.75%			5.75%
Public utilities, actual weighted common equity return, threshold, percentage											6.15%
Public utilities, actual weighted common equity return, threshold for customer refundable fees, refund payments, percentage, tranche one											6.15%
Public utilities, actual weighted common equity return, threshold for customer refundable fees, refund payments, percentage, tranche two											6.65%
Public utilities, actual weighted common equity return, threshold for customer refundable fees, refund payments, percentage, tranche three											7.15%
Public utilities, actual weighted common equity return, threshold, review year with rate RSE upward adjustment, percentage											6.15%
Public utilities, actual weighted common equity return, threshold for customer refundable fees, refund payments, review year with rate RSE upward adjustment, percentage, tranche one											6.15%
Alabama Power | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Weighted cost of equity							6.21%				6.21%			6.21%
Public utilities, actual weighted common equity return, threshold, percentage											7.65%
Public utilities, actual weighted common equity return, threshold for customer refundable fees, refund payments, percentage, tranche one											6.65%
Public utilities, actual weighted common equity return, threshold for customer refundable fees, refund payments, percentage, tranche two											7.15%
Public utilities, actual weighted common equity return, threshold for customer refundable fees, refund payments, percentage, tranche three											7.65%
Customer refundable fees, refund payments, percentage											7.65%
Public utilities, actual weighted common equity return, threshold for customer refundable fees, refund payments, review year with rate RSE upward adjustment, percentage, tranche one											6.90%
Customer refundable fees, refund payments, review year with rate RSE upward adjustment, percentage											6.90%
Alabama Power | Other regulatory assets
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory amortization period											12 months
Under recovered fuel cost												18,000,000
Alabama Power | Unrecovered Investment Costs
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset							$ 649,000,000				$ 649,000,000			$ 649,000,000
Regulatory assets, current							63,000,000				63,000,000			63,000,000
Regulatory assets, noncurrent							586,000,000				586,000,000			586,000,000
Alabama Power | Customer refunds
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities							$ 109,000,000				109,000,000			$ 109,000,000
Alabama Power | Rate ECR refunds
Schedule of Regulatory Assets and Liabilities [Line Items]
Contract with customer, liability, bill credits											$ 31,000,000
Alabama Power | (Over) under recovered regulatory clause revenues
Schedule of Regulatory Assets and Liabilities [Line Items]
Under recovered fuel cost												91,000,000
Year 2020 | Alabama Power
Schedule of Regulatory Assets and Liabilities [Line Items]
PPA, additional generating capacity (in MWs) | MW							240				240			240
Years 2022 Through 2024 | Alabama Power
Schedule of Regulatory Assets and Liabilities [Line Items]
PPA, additional generating capacity (in MWs) | MW							400				400			400
Major storms
Schedule of Regulatory Assets and Liabilities [Line Items]
Storm damage reserves											$ 170,000,000	74,000,000	41,000,000
Major storms | Alabama Power
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset, amortization period							$ 150,000,000				150,000,000			$ 150,000,000
Storm damage reserves							84,000,000				139,000,000	$ 16,000,000	$ 4,000,000
Major storms | Alabama Power | Other regulatory liabilities, current
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset, amortization period							$ 113,000,000				$ 113,000,000			$ 113,000,000
Major storms | Alabama Power | Other regulatory liabilities, current | Scenario, Forecast
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset, amortization period										$ 37,000,000
Restatement Adjustment | Alabama Power
Schedule of Regulatory Assets and Liabilities [Line Items]
Remaining amounts of regulatory assets for which no return on investment during recovery period is provided																		$ 700,000,000
Net capitalized asset retirement costs																		700,000,000
Restatement Adjustment | Alabama Power | Plant Gorgas Units 8, 9, And 10
Schedule of Regulatory Assets and Liabilities [Line Items]
Net investment costs to be transferred upon retirement																		654,000,000
Remaining amounts of regulatory assets for which no return on investment during recovery period is provided																		$ 654,000,000