Asset Retirement Obligations - Fair Value Increases (Decreases) of the Funds (Details) - Securities held in the Funds - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair value increases (decreases)
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Fair value increases (decreases) of the Funds	$ (67)	$ 233	$ 114
Unrealized gains (losses)
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Fair value increases (decreases) of the Funds	(183)	181	48
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	(38)	125	76
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	(96)	98	34
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	(29)	108	38
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	$ (87)	$ 83	$ 14