Regulatory Matters - Alabama Power (Details) - ALABAMA POWER CO $ in Millions	Oct. 01, 2019	Sep. 30, 2019 USD ($) MW	Sep. 06, 2019 MW	May 08, 2019 MW	Apr. 15, 2019 USD ($)
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Additional generating capacity (in MWs)			200
Winter reserve capacity (in MWs)			2,400
Capital investment, amount | $		$ 1,100
PPA, additional generating capacity (in MWs)		640
Plant Barry Unit 8
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Approximate nameplate capacity (MW)				720
Restatement Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Capitalized asset retirement costs, net | $					$ 700
Remaining amounts of regulatory assets for which no return on investment during recovery period is provided | $					700
Restatement Adjustment | Plant Gorgas Units 8, 9, and 10
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net investment costs | $					654
Remaining amounts of regulatory assets for which no return on investment during recovery period is provided | $					$ 654
Subsequent Event
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Regulatory asset, amortization period	5 years
Subsequent Event | Plant Barry Unit 8
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Costs incurred, percentage of estimated in-service costs	5.00%
2020
Error Corrections and Prior Period Adjustments Restatement [Line Items]
PPA, additional generating capacity (in MWs)		240
2022 through 2024
Error Corrections and Prior Period Adjustments Restatement [Line Items]
PPA, additional generating capacity (in MWs)		400