FAIR VALUE MEASUREMENTS - Financial Instruments, Carrying Amount Not Equal to Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amount	$ 52,100	$ 48,300
Fair value	57,100	56,300
Alabama Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amount	9,700	8,900
Fair value	10,900	10,700
Georgia Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amount	13,600	12,800
Fair value	15,100	15,200
Mississippi Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amount	1,500	1,400
Fair value	1,600	1,600
Southern Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amount	3,700	3,700
Fair value	4,100	4,200
Southern Company Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amount	6,900	6,600
Fair value	$ 7,800	$ 8,000