FINANCING - Bank Credit Arrangements Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Feb. 19, 2020	Dec. 31, 2018
Debt Disclosure [Line Items]
Ratio of indebtedness to capitalization, debt covenant, required	70.00%
Amount of variable rate pollution control revenue bonds outstanding requiring liquidity support	$ 1,400,000,000
Remarketed pollution control bonds	275,000,000
Line of credit facility, maximum borrowing capacity	7,608,000,000
Unused credit with banks	$ 7,576,000,000
Other Subsidiaries
Debt Disclosure [Line Items]
Ratio of indebtedness to capitalization, debt covenant, required	65.00%
Line of credit facility, maximum borrowing capacity	$ 30,000,000
Unused credit with banks	30,000,000
Alabama Power
Debt Disclosure [Line Items]
Amount of variable rate pollution control revenue bonds outstanding requiring liquidity support	854,000,000
Remarketed pollution control bonds	87,000,000
Line of credit facility, maximum borrowing capacity	1,328,000,000
Unused credit with banks	1,328,000,000
Georgia Power
Debt Disclosure [Line Items]
Amount of variable rate pollution control revenue bonds outstanding requiring liquidity support	550,000,000
Remarketed pollution control bonds	188,000,000
Line of credit facility, maximum borrowing capacity	1,750,000,000
Unused credit with banks	1,733,000,000
Mississippi Power
Debt Disclosure [Line Items]
Amount of variable rate pollution control revenue bonds outstanding requiring liquidity support	40,000,000
Line of credit facility, maximum borrowing capacity	150,000,000
Unused credit with banks	150,000,000
Southern Power
Debt Disclosure [Line Items]
Line of credit facility, maximum borrowing capacity	600,000,000
Unused credit with banks	591,000,000
Southern Power | Continuing Letter of Credit Facility
Debt Disclosure [Line Items]
Line of credit facility, maximum borrowing capacity	120,000,000
Unused credit with banks	97,000,000		$ 103,000,000
Southern Power | Continuing Letter of Credit Facility | Subsequent Event
Debt Disclosure [Line Items]
Line of credit facility, maximum borrowing capacity		$ 60,000,000
Southern Power | Power Purchase Agreement
Debt Disclosure [Line Items]
Unused credit with banks	23,000,000
Debt instrument, collateral amount	$ 104,000,000		$ 103,000,000