SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Goodwill and Other Intangible Assets and Liabilities (Narrative) (Details) - USD ($)	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Goodwill impairment	$ 0
Goodwill	5,280,000,000	$ 5,280,000,000	$ 5,280,000,000
Intangible liabilities included in other deferred credits and liabilities	816,000,000	696,000,000
Southern Company Gas
Finite-Lived Intangible Assets [Line Items]
Goodwill impairment	0
Goodwill	5,015,000,000	5,015,000,000
Intangible liabilities included in other deferred credits and liabilities	$ 77,000,000	$ 54,000,000
Southern Company Gas | Intangible liabilities
Finite-Lived Intangible Assets [Line Items]
Intangible liabilities included in other deferred credits and liabilities			$ 91,000,000