FAIR VALUE MEASUREMENTS - Financial Instruments, Carrying Amount Not Equal to Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	$ 48,349	$ 44,561
Fair Value	56,264	48,339
Alabama Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	8,864	8,517
Fair Value	10,702	9,525
Georgia Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	12,825	11,660
Fair Value	15,198	12,680
Mississippi Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	1,400	1,589
Fair Value	1,590	1,671
Southern Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	3,692	4,398
Fair Value	4,165	4,708
Southern Company Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	6,626	5,845
Fair Value	$ 7,973	$ 6,509