FAIR VALUE MEASUREMENTS - Financial Instruments, Carrying Amount Not Equal to Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	$ 44,561	$ 45,023
Fair Value	48,339	44,824
Alabama Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	8,517	8,120
Fair Value	9,525	8,370
Georgia Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	11,660	9,838
Fair Value	12,680	9,800
Mississippi Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	1,589	1,579
Fair Value	1,671	1,546
Southern Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	4,398	5,017
Fair Value	4,708	4,980
Southern Company Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Amount	5,845	5,940
Fair Value	$ 6,509	$ 5,965