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RETIREMENT BENEFITS - 401K Plan Matching Contributions (Details) - Employee Savings Plan - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost | $ 119 | $ 120 | $ 113
Alabama Power |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost | 26 | 26 | 25
Georgia Power |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost | 28 | 29 | 27
Mississippi Power |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost | 5 | 5 | 4
Southern Power |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost | 2 | 2 | 2
Southern Company Gas |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost | $ 16 | $ 16 | $ 15

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- Definition Amount of cost for defined contribution plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
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