ASSET RETIREMENT OBLIGATIONS - Fair Value Increases (Decreases) of the Funds (Details) - Securities held in the Funds - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair value increases (decreases)
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Fair value increases (decreases) of the Funds	$ 280	$ 344	$ (67)
Unrealized gains (losses)
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Fair value increases (decreases) of the Funds	220	259	(183)
Alabama Power | Fair value increases (decreases)
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Fair value increases (decreases) of the Funds	142	194	(38)
Alabama Power | Unrealized gains (losses)
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Fair value increases (decreases) of the Funds	121	149	(96)
Georgia Power | Fair value increases (decreases)
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Fair value increases (decreases) of the Funds	138	150	(29)
Georgia Power | Unrealized gains (losses)
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Fair value increases (decreases) of the Funds	$ 99	$ 110	$ (87)