Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - GAS - USD ($) $ in Millions	Total	Paid-In Capital	Retained Earnings (Accumulated Deficit)	Accumulated Other Comprehensive Income (Loss)	SOUTHERN Co GAS	SOUTHERN Co GAS Paid-In Capital	SOUTHERN Co GAS Retained Earnings (Accumulated Deficit)	SOUTHERN Co GAS Accumulated Other Comprehensive Income (Loss)
Beginning balance at Dec. 31, 2023					$ 10,803	$ 10,836	$ (49)	$ 16
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income					409		409
Capital contributions from parent company					2	2
Other comprehensive income					12			12
Cash dividends on common stock	$ (766)		$ (766)		(151)		(151)
Other	7	$ 10	(1)		(1)		(1)
Ending balance at Mar. 31, 2024					11,074	10,838	208	28
Beginning balance at Dec. 31, 2023					10,803	10,836	(49)	16
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	3,867				555
Other comprehensive income					15
Ending balance at Sep. 30, 2024					10,928	10,845	52	31
Beginning balance at Mar. 31, 2024					11,074	10,838	208	28
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income					108		108
Capital contributions from parent company					6	6
Other comprehensive income					7			7
Cash dividends on common stock	(788)		(788)		(151)		(151)
Other	1	3
Ending balance at Jun. 30, 2024					11,044	10,844	165	35
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,535				38		38
Capital contributions from parent company					1	1
Other comprehensive income					(4)			(4)
Cash dividends on common stock	(789)		(789)		(152)		(152)
Other	4	(2)			1		1
Ending balance at Sep. 30, 2024					10,928	10,845	52	31
Beginning balance at Dec. 31, 2024					10,996	10,863	85	48
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income					418		418
Return of capital to parent company					(56)	(56)
Capital contributions from parent company					3	3
Other comprehensive income					12			12
Cash dividends on common stock	(791)		(791)		(149)		(149)
Other	(5)	(1)	(2)		1		1
Ending balance at Mar. 31, 2025					11,225	10,810	355	60
Beginning balance at Dec. 31, 2024					10,996	10,863	85	48
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	3,925				549
Other comprehensive income					(2)
Ending balance at Sep. 30, 2025					11,089	10,854	189	46
Beginning balance at Mar. 31, 2025					11,225	10,810	355	60
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income					106		106
Capital contributions from parent company					23	23
Other comprehensive income					(7)			(7)
Cash dividends on common stock	(815)		(815)		(148)		(148)
Other	6	6	1		(1)		(1)
Ending balance at Jun. 30, 2025					11,198	10,833	312	53
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,711				25		25
Capital contributions from parent company					21	21
Other comprehensive income					(7)			(7)
Cash dividends on common stock	(815)		(815)		(149)		(149)
Other	$ (4)	$ (10)	$ 1	$ (1)	1		1
Ending balance at Sep. 30, 2025					$ 11,089	$ 10,854	$ 189	$ 46