FAIR VALUE MEASUREMENTS - Financial Instruments, Carrying Amount Not Equal to Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amount	$ 54,600	$ 52,100
Fair value	48,600	57,100
Alabama Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amount	10,600	9,700
Fair value	9,200	10,900
Georgia Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amount	14,700	13,600
Fair value	13,000	15,100
Mississippi Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amount	1,500	1,500
Fair value	1,300	1,600
Southern Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amount	3,000	3,700
Fair value	2,800	4,100
Southern Company Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amount	7,400	6,900
Fair value	$ 6,500	$ 7,800