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LEASES - Leveraged Leases (Narrative) (Details) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2023 leveragedLease | Dec. 31, 2021 project | Dec. 31, 2020 leveragedLease project
Leveraged Lease [Line Items] |  |  |
Number of leveraged leases | leveragedLease | 1 |  | 4
Number of domestic projects subject to leveraged lease |  |  | 2
Number of international projects subject to leveraged lease |  |  | 2
Number of domestic projects subject to leveraged lease sold |  | 1 |
Remaining lease term | 8 years |  |

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- Definition Leveraged Lease Line Items. + References No definition available. + Details
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Namespace Prefix: | so_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Leveraged Leases, Remaining Lease Term + References No definition available. + Details
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Data Type: | xbrli:durationItemType
Balance Type: | na
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- Definition Number of Domestic Projects Subject to Leveraged Lease + References No definition available. + Details
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Data Type: | xbrli:integerItemType
Balance Type: | na
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- Definition Number of International Projects Subject to Leveraged Lease + References No definition available. + Details
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Namespace Prefix: | so_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition Number Of Leveraged Leases + References No definition available. + Details
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Namespace Prefix: | so_
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- Definition Number of Projects Subject to Leveraged Lease Sold + References No definition available. + Details
Name: | so_NumberOfProjectsSubjectToLeveragedLeaseSold
Namespace Prefix: | so_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration