REGULATORY MATTERS - MPC 2019 Base Rate Case and Performance Evaluation Plan (Details) - Mississippi Power $ in Millions				1 Months Ended	12 Months Ended
	Dec. 07, 2021 USD ($)	Jun. 08, 2021 USD ($)	Apr. 06, 2021 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2021 filing
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount	$ 9.0	$ 16.0	$ 28.0	$ (16.7)
Approved rate decrease, percent		(1.80%)		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
Performance evaluation plan, number of filings per calendar year | filing					2
Weighted Average | Measurement Input, Average Customer Price
Public Utilities, General Disclosures [Line Items]
Performance evaluation plan, measurement input					0.40
Weighted Average | Measurement Input, Service Reliability
Public Utilities, General Disclosures [Line Items]
Performance evaluation plan, measurement input					0.40
Weighted Average | Measurement Input, Customer Satisfaction
Public Utilities, General Disclosures [Line Items]
Performance evaluation plan, measurement input					0.20