Derivatives - Narrative (Details) MMBTU in Millions	9 Months Ended
	Sep. 30, 2019 USD ($) MMBTU	Dec. 31, 2018 USD ($)
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	25
Cash flow hedge gain (loss) to be reclassified within twelve months	$ (21,000,000)
Collateral already posted, aggregate fair value	$ 166,000,000
ALABAMA POWER CO
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	4
Cash flow hedge gain (loss) to be reclassified within twelve months	$ 0
GEORGIA POWER CO
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	8
Cash flow hedge gain (loss) to be reclassified within twelve months	$ 0
MISSISSIPPI POWER CO
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	4
Cash flow hedge gain (loss) to be reclassified within twelve months	$ 0
SOUTHERN POWER CO
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	9
Cash flow hedge gain (loss) to be reclassified within twelve months	$ 0
SOUTHERN Co GAS
Derivative [Line Items]
Cash flow hedge gain (loss) to be reclassified within twelve months	0
Collateral already posted, aggregate fair value	166,000,000	$ 277,000,000
Parent Company and Southern Power
Derivative [Line Items]
Foreign currency cash flow hedge gain (loss) to be reclassified during next 12 months	(24,000,000)
Registrants | Derivative Counterparties
Derivative [Line Items]
Collateral already posted, aggregate fair value	$ 0