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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Storm Damage Reserves (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Alabama Power |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Additional accruals authorized by state PSCs | $ 65.0 | $ 100.0 | $ 84.0
Georgia Power |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Storm damage reserves | 213.0 |  |
Major storms |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Storm damage reserves | 286.0 | 326.0 | 170.0
Major storms | Alabama Power |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Additional accruals authorized by state PSCs | 65.0 | 100.0 | 84.0
Storm damage reserves | 75.0 | 112.0 | 139.0
Major storms | Georgia Power |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Storm damage reserves |  | 213.0 | 30.0
Major storms | Mississippi Power |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Storm damage reserves | $ (1.8) | 0.8 | $ 1.4
Accumulated Deferred Income Taxes | Major storms | Alabama Power |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Storm damage reserves |  | $ 39.0 |

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- Definition Amount, after effects of reinsurance, of increase (decrease) in the liability for unpaid claims and claims adjustment expense. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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