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Regulatory Matters - GAS Schedule of Infrastructure Replacement Programs and Capital Projects (Details) - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2023 | Dec. 31, 2019
SOUTHERN Co GAS |  |
Public Utilities, General Disclosures [Line Items] |  |
Capital expenditures | $ 294 |
Investing in Illinois | SOUTHERN Co GAS |  |
Public Utilities, General Disclosures [Line Items] |  |
Capital expenditures | 196 |
Investing in Illinois | Nicor Gas |  |
Public Utilities, General Disclosures [Line Items] |  |
Capital expenditures |  | $ 415
SAVE | SOUTHERN Co GAS |  |
Public Utilities, General Disclosures [Line Items] |  |
Capital expenditures | 37 |
System Reinforcement Rider | SOUTHERN Co GAS |  |
Public Utilities, General Disclosures [Line Items] |  |
Capital expenditures | 57 |
Pipeline Replacement Program | SOUTHERN Co GAS |  |
Public Utilities, General Disclosures [Line Items] |  |
Capital expenditures | $ 4 |

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