Contingencies and Regulatory Matters - Textual - SO Regulatory Matters APC (Details) - ALABAMA POWER CO			1 Months Ended	12 Months Ended
	Jan. 01, 2019 $ / KWH_Kilowatt_hour	Jan. 01, 2018 $ / KWH_Kilowatt_hour	Apr. 30, 2016 MW	Dec. 31, 2017 USD ($)	Dec. 31, 2015 MW	Feb. 17, 2017 USD ($)	Jan. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2014
Schedule of Regulatory Assets and Liabilities [Line Items]
Adjusting point of weighted cost of equity				5.98%
Percent of basis points				0.07%
Percent of designated customer value benchmark survey				33.33%					33.33%
Rate adjustment period				2 years
Maximum percentage of rate RSE				4.00%
Maximum annual percentage of ratio R\rate				5.00%
Rate RSE refund liability								$ 73,000,000
Rate RSE increase							4.48%
Rate RSE increase amount							$ 245,000,000
Tax cuts and jobs act, anticipated retail revenue reduction, next twelve months				$ 250,000,000
Under recovered certified power purchase agreements				12,000,000				142,000,000
Over (under) recovered environmental clause				17,000,000				9,000,000
Over recovered fuel cost				25,000,000				$ 76,000,000
Minimum natural disaster reserve balance, triggering establishment charge				50,000,000
Natural disaster reserve, expected collection annually until reserve is restored				16,000,000
Natural disaster reserve authorized limit				$ 75,000,000
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
Natural disaster reserve				$ 38,000,000
Minimum
Schedule of Regulatory Assets and Liabilities [Line Items]
Weighted cost of equity				5.75%
Potential period for next depreciation study				2 years
Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Weighted cost of equity				6.21%
Potential period for next depreciation study				4 years
Regulatory Assets
Schedule of Regulatory Assets and Liabilities [Line Items]
Under recovered certified power purchase agreements				$ 69,000,000
Over (under) recovered environmental clause						$ (36,000,000)
Plant Gorgas Units 6 and 7
Schedule of Regulatory Assets and Liabilities [Line Items]
Capacity of units included in request for decertification of units | MW					200
Plant Barry Unit 3
Schedule of Regulatory Assets and Liabilities [Line Items]
Capacity of units included in request for decertification of units | MW					225
Plant Barry Units 1 And 2
Schedule of Regulatory Assets and Liabilities [Line Items]
Capacity of units included in request for decertification of units | MW					250
Plant Greene County Units 1 And 2
Schedule of Regulatory Assets and Liabilities [Line Items]
Capacity of units included in request for decertification of units | MW			300
Subsequent Event
Schedule of Regulatory Assets and Liabilities [Line Items]
Current billing rates under rate ECR in terms of per units | $ / KWH_Kilowatt_hour		0.02015
Scenario, Forecast
Schedule of Regulatory Assets and Liabilities [Line Items]
Current billing rates under rate ECR in terms of per units | $ / KWH_Kilowatt_hour	0.05910