SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Concentration of Revenue (Narrative) (Details) - Mississippi Power	12 Months Ended
Dec. 31, 2021
Concentration Risk [Line Items]
Wholesale customers representing total operating revenues	14.30%
Period of cancellation notices	10 years

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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