Contingencies and Regulatory Matters - Alabama Power (Details)				1 Months Ended		3 Months Ended	9 Months Ended	12 Months Ended
	Aug. 06, 2018	May 01, 2018 USD ($) $ / KWH_Kilowatt_hour	Apr. 30, 2018 $ / KWH_Kilowatt_hour	Jan. 31, 2019 $ / KWH_Kilowatt_hour	Jun. 30, 2018	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2025	Dec. 31, 2018 USD ($)	Dec. 31, 2017
ALABAMA POWER CO
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage							47.00%
Public utilities, allowed weighted common equity return, percentage					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 three, percentage							75.00%
Customer refund							$ 50,000,000
Customer refundable fees, refund payments							151,000,000
Current billing rates under rate ecr in terms of per units | $ / KWH_Kilowatt_hour		2.353	2.015
Approved increase (decrease) in fuel rates amount		$ 100,000,000
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							$ 30,000,000
ALABAMA POWER CO | Minimum
Loss Contingencies [Line Items]
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%
ALABAMA POWER CO | Maximum
Loss Contingencies [Line Items]
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%
ALABAMA POWER CO | Scenario, Forecast
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage								55.00%
Customer refundable fees, refund payments						$ 257,000,000
Current billing rates under rate ecr in terms of per units | $ / KWH_Kilowatt_hour				5.910
Tax Cuts and Jobs Act of 2017, change in tax rate, deferred tax liability, income tax benefit									$ 50,000,000
Reserve Margin Plan | Plant Greene County Units 1 and 2 | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Accelerated retirement period	4 years