SEGMENT AND RELATED INFORMATION - Gas Revenues (Details)	3 Months Ended		12 Months Ended
	Jun. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2025 USD ($) segment state	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Segment Reporting Information [Line Items]
Operating revenues			$ 29,553,000,000	$ 26,724,000,000	$ 25,253,000,000
Other segment items			15,757,000,000	13,995,000,000	13,954,000,000
Depreciation and amortization			5,501,000,000	4,755,000,000	4,525,000,000
Earnings from equity method investments			112,000,000	139,000,000	144,000,000
Interest expense			3,238,000,000	2,743,000,000	2,446,000,000
Income taxes (benefit)			828,000,000	969,000,000	496,000,000
Segment net income (loss)			4,341,000,000	4,401,000,000	3,976,000,000
Total assets at December 31, 2025		$ 139,331,000,000	$ 155,720,000,000	145,180,000,000	139,331,000,000
Southern Company Gas
Segment Reporting Information [Line Items]
Number of reportable segments | segment			3
Number of states in which entity operates | state			4
Operating revenues			$ 5,044,000,000	4,456,000,000	4,702,000,000
Other segment items			3,172,000,000	2,613,000,000	3,124,000,000
Depreciation and amortization			708,000,000	650,000,000	582,000,000
Earnings from equity method investments			127,000,000	146,000,000	140,000,000
Interest expense			377,000,000	341,000,000	310,000,000
Income taxes (benefit)			182,000,000	258,000,000	211,000,000
Segment net income (loss)			732,000,000	740,000,000	615,000,000
Total assets at December 31, 2025		25,083,000,000	27,387,000,000	26,177,000,000	25,083,000,000
Estimated loss on regulatory disallowance			$ (63,000,000)	0	(96,000,000)
Southern Company Gas | Southern Natural Gas Company, LLC
Segment Reporting Information [Line Items]
Ownership percentage, equity method investment			50.00%
Southern Company Gas | Dalton Pipeline
Segment Reporting Information [Line Items]
Ownership percentage, equity method investment			50.00%
Nicor Gas
Segment Reporting Information [Line Items]
Estimated loss on regulatory disallowance	$ (38,000,000)	(58,000,000)	$ 63,000,000		96,000,000
Estimated loss on regulatory disallowance, after tax	$ (28,000,000)	(44,000,000)	47,000,000		72,000,000
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			28,821,000,000	26,059,000,000	24,700,000,000
Other segment items			15,039,000,000	13,342,000,000	13,405,000,000
Depreciation and amortization			5,433,000,000	4,684,000,000	4,447,000,000
Earnings from equity method investments			133,000,000	154,000,000	139,000,000
Interest expense			1,822,000,000	1,713,000,000	1,584,000,000
Income taxes (benefit)			1,221,000,000	1,261,000,000	794,000,000
Segment net income (loss)			5,439,000,000	5,213,000,000	4,609,000,000
Total assets at December 31, 2025		137,728,000,000	153,416,000,000	143,382,000,000	137,728,000,000
Operating Segments | Southern Company Gas
Segment Reporting Information [Line Items]
Operating revenues			5,042,000,000	4,447,000,000	4,685,000,000
Other segment items			3,170,000,000	2,598,000,000	3,109,000,000
Depreciation and amortization			706,000,000	649,000,000	581,000,000
Earnings from equity method investments			127,000,000	146,000,000	140,000,000
Interest expense			381,000,000	349,000,000	310,000,000
Income taxes (benefit)			171,000,000	244,000,000	195,000,000
Segment net income (loss)			741,000,000	753,000,000	630,000,000
Total assets at December 31, 2025		26,055,000,000	28,615,000,000	27,336,000,000	26,055,000,000
All Other
Segment Reporting Information [Line Items]
Operating revenues			893,000,000	843,000,000	718,000,000
Other segment items			872,000,000	802,000,000	699,000,000
Depreciation and amortization			68,000,000	71,000,000	78,000,000
Earnings from equity method investments			(21,000,000)	(17,000,000)	5,000,000
Interest expense			1,416,000,000	1,030,000,000	879,000,000
Income taxes (benefit)			(393,000,000)	(292,000,000)	(298,000,000)
Segment net income (loss)			(1,091,000,000)	(785,000,000)	(635,000,000)
Total assets at December 31, 2025		2,446,000,000	2,829,000,000	2,371,000,000	2,446,000,000
All Other | Southern Company Gas
Segment Reporting Information [Line Items]
Operating revenues			12,000,000	23,000,000	36,000,000
Other segment items			12,000,000	29,000,000	34,000,000
Depreciation and amortization			2,000,000	1,000,000	1,000,000
Earnings from equity method investments			0	0	0
Interest expense			(4,000,000)	(8,000,000)	0
Income taxes (benefit)			11,000,000	14,000,000	16,000,000
Segment net income (loss)			(9,000,000)	(13,000,000)	(15,000,000)
Total assets at December 31, 2025		9,675,000,000	10,643,000,000	10,047,000,000	9,675,000,000
Eliminations
Segment Reporting Information [Line Items]
Operating revenues			(161,000,000)	(178,000,000)	(165,000,000)
Other segment items			(154,000,000)	(149,000,000)	(150,000,000)
Depreciation and amortization			0	0	0
Earnings from equity method investments			0	2,000,000	0
Interest expense			0	0	(17,000,000)
Income taxes (benefit)			0	0	0
Segment net income (loss)			(7,000,000)	(27,000,000)	2,000,000
Total assets at December 31, 2025		(843,000,000)	(525,000,000)	(573,000,000)	(843,000,000)
Eliminations | Southern Company Gas
Segment Reporting Information [Line Items]
Operating revenues			(10,000,000)	(14,000,000)	(19,000,000)
Other segment items			(10,000,000)	(14,000,000)	(19,000,000)
Depreciation and amortization			0	0	0
Earnings from equity method investments			0	0	0
Interest expense			0	0	0
Income taxes (benefit)			0	0	0
Segment net income (loss)			0	0	0
Total assets at December 31, 2025		(10,647,000,000)	(11,871,000,000)	(11,206,000,000)	(10,647,000,000)
Gas distribution operations | Operating Segments | Southern Company Gas
Segment Reporting Information [Line Items]
Operating revenues			4,428,000,000	3,899,000,000	4,105,000,000
Other segment items			2,721,000,000	2,236,000,000	2,702,000,000
Depreciation and amortization			687,000,000	630,000,000	561,000,000
Earnings from equity method investments			0	0	0
Interest expense			342,000,000	311,000,000	275,000,000
Income taxes (benefit)			109,000,000	172,000,000	126,000,000
Segment net income (loss)			569,000,000	550,000,000	441,000,000
Total assets at December 31, 2025		22,906,000,000	25,391,000,000	24,067,000,000	22,906,000,000
Gas Pipeline Investments | Operating Segments | Southern Company Gas
Segment Reporting Information [Line Items]
Operating revenues			32,000,000	32,000,000	32,000,000
Other segment items			5,000,000	6,000,000	5,000,000
Depreciation and amortization			5,000,000	5,000,000	5,000,000
Earnings from equity method investments			127,000,000	146,000,000	140,000,000
Interest expense			36,000,000	35,000,000	32,000,000
Income taxes (benefit)			28,000,000	31,000,000	32,000,000
Segment net income (loss)			85,000,000	101,000,000	98,000,000
Total assets at December 31, 2025		1,534,000,000	1,475,000,000	1,573,000,000	1,534,000,000
Gas marketing services | Operating Segments | Southern Company Gas
Segment Reporting Information [Line Items]
Operating revenues			582,000,000	516,000,000	548,000,000
Other segment items			444,000,000	356,000,000	402,000,000
Depreciation and amortization			14,000,000	14,000,000	15,000,000
Earnings from equity method investments			0	0	0
Interest expense			3,000,000	3,000,000	3,000,000
Income taxes (benefit)			34,000,000	41,000,000	37,000,000
Segment net income (loss)			87,000,000	102,000,000	91,000,000
Total assets at December 31, 2025		1,615,000,000	1,749,000,000	1,696,000,000	1,615,000,000
Southern Company Gas
Segment Reporting Information [Line Items]
Operating revenues			5,044,000,000	4,456,000,000	4,702,000,000
Southern Company Gas | Gas distribution operations
Segment Reporting Information [Line Items]
Operating revenues			4,428,000,000	3,899,000,000	4,090,000,000
Southern Company Gas | Gas marketing services
Segment Reporting Information [Line Items]
Operating revenues			582,000,000	516,000,000	548,000,000
Southern Company Gas | Other Gas Revenue
Segment Reporting Information [Line Items]
Operating revenues			34,000,000	41,000,000	64,000,000
Southern Company Gas | Operating Segments
Segment Reporting Information [Line Items]
Operating revenues			5,044,000,000	4,456,000,000	4,702,000,000
Other segment items			3,172,000,000	2,613,000,000	3,124,000,000
Depreciation and amortization			708,000,000	650,000,000	582,000,000
Earnings from equity method investments			127,000,000	146,000,000	140,000,000
Interest expense			377,000,000	341,000,000	310,000,000
Income taxes (benefit)			182,000,000	258,000,000	211,000,000
Segment net income (loss)			732,000,000	740,000,000	615,000,000
Total assets at December 31, 2025		$ 25,083,000,000	$ 27,387,000,000	$ 26,177,000,000	$ 25,083,000,000