SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenues (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 29,553	$ 26,724	$ 25,253
Southern Company Gas
Disaggregation of Revenue [Line Items]
Period for collection of revenue prior to billings	24 months
Total operating revenues	$ 5,044	4,456	4,702
Southern Company Gas | Alternative revenue programs
Disaggregation of Revenue [Line Items]
Total operating revenues	$ (53)	$ 43	$ 20