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REGULATORY MATTERS - GAS Schedule of Infrastructure Replacement Programs and Capital Projects (Details) - Southern Company Gas $ in Millions | 12 Months Ended
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Dec. 31, 2021 USD ($) mi | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Public Utilities, General Disclosures [Line Items] |  |  |
Expenditures in 2021 | $ 461 |  |
Expenditures Since Project Inception | $ 2,852 |  |
Pipe Installed Since Project Inception (in miles) | mi | 1,628 |  |
Scope of Program (in miles) | mi | 2,107 |  |
Investing In Illinois |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Expenditures in 2021 | $ 408 |  |
Expenditures Since Project Inception | $ 2,508 |  |
Pipe Installed Since Project Inception (in miles) | mi | 1,153 |  |
Scope of Program (in miles) | mi | 1,394 |  |
Program Duration | 9 years |  |
Steps to Advance Virginia's Energy (SAVE) |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Expenditures in 2021 | $ 51 | $ 49 | $ 45
Expenditures Since Project Inception | $ 342 |  |
Pipe Installed Since Project Inception (in miles) | mi | 470 |  |
Scope of Program (in miles) | mi | 640 |  |
Program Duration | 13 years |  |
System Reinforcement Rider |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Expenditures in 2021 | $ 0 |  |
Expenditures Since Project Inception | $ 0 |  |
Program Duration | 3 years |  |
Pipeline Replacement Program |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Expenditures in 2021 | $ 2 |  |
Expenditures Since Project Inception | $ 2 |  |
Pipe Installed Since Project Inception (in miles) | mi | 5 |  |
Scope of Program (in miles) | mi | 73 |  |
Program Duration | 7 years |  |

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