DERIVATIVES - Foreign Currency Derivatives (Details) € in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2020 EUR (€)	Dec. 31, 2019 USD ($)
Foreign currency derivatives | Cash Flow Hedges of Existing Debt
Derivative [Line Items]
Pay Notional	$ 1,241
Receive Notional | €		€ 1,100
Fair Value Gain (Loss) December 31, 2020	65
Southern Power
Derivative [Line Items]
Receive Notional	1,100
Fair Value Gain (Loss) December 31, 2020	109		$ (5)
Southern Power | Foreign currency derivatives | June 2022 | Cash Flow Hedges of Existing Debt
Derivative [Line Items]
Pay Notional	$ 677
Pay Rate	2.95%
Receive Notional | €		600
Receive Rate	1.00%
Fair Value Gain (Loss) December 31, 2020	$ 40
Southern Power | Foreign currency derivatives | June 2026 | Cash Flow Hedges of Existing Debt
Derivative [Line Items]
Pay Notional	$ 564
Pay Rate	3.78%
Receive Notional | €		€ 500
Receive Rate	1.85%
Fair Value Gain (Loss) December 31, 2020	$ 25