REGULATORY MATTERS - Mississippi Power (Details) - MISSISSIPPI POWER CO - USD ($) $ in Millions	Jun. 25, 2020	Mar. 17, 2020	Sep. 30, 2020
Loss Contingencies [Line Items]
Decrease in retail rates, percentage		1.85%
Base rate case filing, measurement input, average equity ratio		53.00%
Base rate case filing, measurement input, maximum equity ratio		55.00%
Base rate case filing, measurement input, return on investment		7.57%
Regulatory asset, amortization period		4 years
Regulatory liability, amortization period		3 years
Annual base rate adjustment, increase in depreciation		$ 10.0
Annual base rate adjustment, compensation costs excluded		3.9
Incremental COVID-19 costs deferred			$ 2.0
Increase in retail rates	$ 2.0	$ (16.7)