Regulatory Matters - APC Regulatory Matters (Details) - Alabama Power MW in Thousands		1 Months Ended	12 Months Ended
	May 01, 2018 USD ($) $ / KWH_Kilowatt_hour	Jan. 31, 2019	Dec. 31, 2025	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) $ / KWH_Kilowatt_hour	Dec. 31, 2017 USD ($) $ / KWH_Kilowatt_hour	Feb. 19, 2019 USD ($) MW	Feb. 06, 2019 USD ($)	Feb. 05, 2019 USD ($)	Feb. 28, 2017 USD ($)	Jan. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Schedule of Regulatory Assets and Liabilities [Line Items]
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%
Percent of basis points					0.07%
Percent of designated customer value benchmark survey					33.33%
Rate RSE refund liability												$ 73,000,000
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					47.00%
Weighted common equity return						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%
Under recovered certified power purchase agreements					$ 25,000,000	$ 12,000,000
Over (under) recovered environmental clause					$ (42,000,000)	$ (17,000,000)
Current billing rates under rate ECR (usd per KWH) | $ / KWH_Kilowatt_hour	0.02353				0.05910	0.02015
Under recovered fuel cost					$ 109,000,000	$ 25,000,000
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					63,000,000
Amount offset under recovered balance					30,000,000
Amount remaining for benefit of customers					33,000,000
Minimum natural disaster reserve balance, triggering establishment charge						50,000,000
Natural disaster reserve authorized limit					75,000,000	75,000,000
Natural disaster reserve, expected collection annually until reserve is restored						$ 16,000,000
Natural disaster reserve					$ 20,000,000
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
Maximum rate NDR charge per month, monthly residential customer account					5
Scenario, Forecast
Schedule of Regulatory Assets and Liabilities [Line Items]
Contract with customer, liability, bill credits				$ 50,000,000
Public utilities, approved return on equity			55.00%
Weighted common equity return		6.15%
Retail revenue requirement for environmental compliance				205,000,000
Rate case settlement annual increase base rates				183,000,000
Regulatory Assets
Schedule of Regulatory Assets and Liabilities [Line Items]
Under recovered certified power purchase agreements					$ 69,000,000							$ 142,000,000
Over (under) recovered environmental clause										$ (36,000,000)
Minimum
Schedule of Regulatory Assets and Liabilities [Line Items]
Weighted cost of equity					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%
Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Weighted cost of equity					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%
Other regulatory assets
Schedule of Regulatory Assets and Liabilities [Line Items]
Under recovered fuel cost					$ 18,000,000
Unrecovered Investment Costs
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset					42,000,000
Regulatory assets, current					10,000,000
Regulatory assets, noncurrent					32,000,000
Rate RSE refund
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities					109,000,000
Rate ECR refunds | Scenario, Forecast
Schedule of Regulatory Assets and Liabilities [Line Items]
Contract with customer, liability, bill credits				$ 34,000,000
Under/(over) recovered regulatory clause revenues
Schedule of Regulatory Assets and Liabilities [Line Items]
Under recovered fuel cost					$ 91,000,000
Subsequent Event
Schedule of Regulatory Assets and Liabilities [Line Items]
Rate RSE refund liability								$ 75,000,000	$ 75,000,000
Subsequent Event | Plant Gorgas Units 8, 9, And 10
Schedule of Regulatory Assets and Liabilities [Line Items]
Capacity of retired plants (mw) | MW							1
Net investment costs to be transferred upon retirement							$ 740,000,000