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REGULATORY MATTERS - GAS Schedule of Infrastructure Replacement Programs and Capital Projects (Details) - Southern Company Gas $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 USD ($) mi | Dec. 31, 2022 USD ($)
Public Utilities, General Disclosures [Line Items] |  |
Capital Expenditures in 2023 | $ 553 |
Capital Expenditures Since Project Inception | $ 3,910 |
Pipe Installed Since Project Inception (in miles) | mi | 1,969 |
Scope of Program (in miles) | mi | 2,135 |
Investing In Illinois Qualifying Infrastructure Plant |  |
Public Utilities, General Disclosures [Line Items] |  |
Capital Expenditures in 2023 | $ 365 |
Capital Expenditures Since Project Inception | $ 3,228 |
Pipe Installed Since Project Inception (in miles) | mi | 1,367 |
Scope of Program (in miles) | mi | 1,367 |
Program Duration | 9 years |
SAVE |  |
Public Utilities, General Disclosures [Line Items] |  |
Capital Expenditures in 2023 | $ 75 |
Capital Expenditures Since Project Inception | $ 486 |
Pipe Installed Since Project Inception (in miles) | mi | 567 |
Scope of Program (in miles) | mi | 695 |
Program Duration | 13 years |
System Reinforcement Rider |  |
Public Utilities, General Disclosures [Line Items] |  |
Capital Expenditures in 2023 | $ 104 | $ 76
Capital Expenditures Since Project Inception | $ 180 |
Pipe Installed Since Project Inception (in miles) | mi | 20 |
Program Duration | 3 years |
Pipeline Replacement Program |  |
Public Utilities, General Disclosures [Line Items] |  |
Capital Expenditures in 2023 | $ 9 |
Capital Expenditures Since Project Inception | $ 16 |
Pipe Installed Since Project Inception (in miles) | mi | 15 |
Scope of Program (in miles) | mi | 73 |
Program Duration | 7 years |

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