CONTINGENCIES, COMMITMENTS, AND GUARANTEES - MPC Other Matters (Details)			12 Months Ended			60 Months Ended
	Dec. 31, 2019	Jan. 01, 2019 USD ($)	Dec. 31, 2020 USD ($) MW	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2025 USD ($)	Jan. 15, 2024 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2010 USD ($)
Loss Contingencies [Line Items]
Period costs			$ 19,000,000	$ 37,000,000
Mississippi Power
Loss Contingencies [Line Items]
Plant capacity under coal gasification combined cycle technology | MW			582
Period costs			$ 0	1,000,000
Kemper County energy facility assets, net | Mississippi Power
Loss Contingencies [Line Items]
Reduction in customer rates annually					$ 26,800,000
Pre-tax charge to income			4,000,000	$ 24,000,000	37,000,000
After tax charge to income			$ 3,000,000		68,000,000
Firm transportation agreement, term	15 years
Government grants expected								$ 137,000,000	$ 270,000,000
Government grants received					$ 387,000,000
Scenario, Forecast | Kemper County energy facility assets, net | Mississippi Power | Minimum
Loss Contingencies [Line Items]
Period costs						$ 10,000,000
Scenario, Forecast | Kemper County energy facility assets, net | Mississippi Power | Maximum
Loss Contingencies [Line Items]
Period costs						$ 20,000,000
Scenario, Forecast | Plant Daniel | Mississippi Power
Loss Contingencies [Line Items]
Payments to acquire businesses							$ 1
Business combination, option to purchase, period							120 days
Scenario, Forecast | Plant Daniel | Gulf Power
Loss Contingencies [Line Items]
Undivided ownership interest to be restructured							50.00%
Percent ownership in individual unit							100.00%
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Gulf Power
Loss Contingencies [Line Items]
Proceeds from divestiture of businesses, amount held back		$ 75,000,000