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SEGMENT AND RELATED INFORMATION - Gas Revenues (Details) - USD ($) $ in Millions |  | 1 Months Ended | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | ---
Jul. 01, 2021 | Dec. 31, 2020 | Sep. 30, 2021 | Jun. 30, 2021 | Dec. 31, 2021 | Dec. 31, 2021 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  | $ 23,113 | $ 20,375 | $ 21,419
Impairment charges |  |  |  |  |  |  | 2 | 0 | 168
Gain on dispositions, net |  |  |  |  |  |  | 176 | 66 | 2,588
Southern Company Gas |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  | 4,380 | 3,434 | 3,792
Impairment charges |  |  |  |  |  |  | 0 | 0 | 115
Gain on dispositions, net |  |  |  |  |  |  | 127 | 22 | 0
Southern Company Gas | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Sequent |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |
Gain on dispositions, net | $ 121 |  |  |  |  |  | 121 |  |
Gain (loss) on dispositions, after tax | $ 92 |  |  |  |  |  | 92 |  |
Deferred tax expense, increase from change in appointment rate |  |  |  |  | $ 85 | $ 85 | 85 |  |
Southern Company Gas | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Jefferson Island | Natural Gas Storage Facility |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |
Impairment charges |  |  |  |  |  |  |  | 91 |
Impairment charges, after tax |  |  |  |  |  |  |  | 69 |
Gain on dispositions, net |  | $ 22 |  |  |  |  |  |  |
Gain (loss) on dispositions, after tax |  | $ 16 |  |  |  |  |  |  |
Southern Company Gas | PennEast Pipeline |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |
Impairment charges |  |  | $ 2 | $ 82 |  |  | 84 |  |
Impairment charges, after tax |  |  | $ 2 | $ 58 |  |  | 67 |  |
Operating Segments | Southern Company Gas |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  | 4,374 | 3,466 | 3,810
Southern Company Gas |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  | 4,380 | 3,434 | 3,792
Southern Company Gas | Gas distribution operations |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  | 3,656 | 2,902 | 3,001
Southern Company Gas | Gas marketing services |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  | 475 | 408 | 456
Southern Company Gas | Other Gas Revenue |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  | 249 | 124 | 335
Southern Company Gas | Operating Segments |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  | 4,380 | 3,434 | 3,792
Wholesale gas services | Operating Segments | Southern Company Gas |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  | 188 | 74 | 294
Less Gross Gas Costs |  |  |  |  |  |  | 3,783 | 4,585 | 5,684
Wholesale gas services | Operating Segments | Third Party Gross Revenues | Southern Company Gas |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  | 3,881 | 4,544 | 5,703
Wholesale gas services | Operating Segments | Intercompany Revenues | Southern Company Gas |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  | 90 | 115 | 275
Wholesale gas services | Operating Segments | Total Gross Revenues | Southern Company Gas |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  | $ 3,971 | $ 4,659 | $ 5,978

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