SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenues (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 25,253	$ 29,279	$ 23,113
Southern Company Gas
Disaggregation of Revenue [Line Items]
Period for collection of revenue prior to billings	24 months
Total operating revenues	$ 4,702	5,962	4,380
Southern Company Gas | Alternative revenue programs
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 20	$ (5)	$ 11