SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Goodwill and Other Intangible Assets and Liabilities (Narrative) (Details) - USD ($)	3 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Goodwill impairment	$ 119,000,000
Goodwill	5,161,000,000	$ 5,280,000,000	$ 5,280,000,000
Intangible liabilities included in other deferred credits and liabilities	1,167,000,000	816,000,000
PowerSecure
Finite-Lived Intangible Assets [Line Items]
Goodwill	144,000,000
Southern Company Gas
Finite-Lived Intangible Assets [Line Items]
Goodwill	5,015,000,000	5,015,000,000
Intangible liabilities included in other deferred credits and liabilities	$ 179,000,000	$ 77,000,000