DERIVATIVES - Foreign Currency Derivatives (Details) - 12 months ended Dec. 31, 2019 - Foreign currency derivatives - Cash Flow Hedges of Existing Debt € in Millions, $ in Millions	USD ($)	EUR (€)
Derivative [Line Items]
Pay Notional	$ 1,241
Receive Notional | €		€ 1,100
Fair Value Gain (Loss) December 31, 2019	(8)
Southern Power | Maturity Date June 2022
Derivative [Line Items]
Pay Notional	$ 677
Pay Rate	2.95%
Receive Notional | €		600
Receive Rate	1.00%
Fair Value Gain (Loss) December 31, 2019	$ (7)
Southern Power | Maturity Date June 2026
Derivative [Line Items]
Pay Notional	$ 564
Pay Rate	3.78%
Receive Notional | €		€ 500
Receive Rate	1.85%
Fair Value Gain (Loss) December 31, 2019	$ (1)