REGULATORY MATTERS - MPC Fuel Cost Recovery and Ad Valorem Tax Adjustment (Details) - Mississippi Power - USD ($) $ in Millions				1 Months Ended
	Dec. 07, 2021	Jun. 08, 2021	Apr. 06, 2021	Feb. 28, 2022	Jan. 31, 2022	Feb. 28, 2021	Jan. 31, 2021	Mar. 31, 2020	Feb. 29, 2020	Jan. 31, 2020	Feb. 28, 2019	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount	$ 9.0	$ 16.0	$ 28.0					$ (16.7)
PSC retail increase													$ 10.0	$ 2.0
Public utilities, valorem taxes previously recovered, amount			$ 19.0
Retail electric revenues
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount						$ 2.0			$ (24.0)		$ (35.0)
Over recovered fuel cost												$ 4.0	24.0
Retail electric revenues | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount				$ 43.0
MRA Revenue
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount							$ 5.0			$ 1.0
Over recovered fuel cost													$ 10.0
MRA Revenue | Subsequent Event
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount					$ 11.0