Derivatives - Narrative (Details) MWh in Millions, MMBTU in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) MMBTU MWh	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints	23
Estimated pre-tax gains (losses) that will be reclassified from OCI to interest expense for the next 12-month period | $	$ (19)
Collateral already posted, aggregate fair value | $	$ 277	$ 193
Energy-related, Natural Gas
Derivative [Line Items]
Net Purchased mmBtu	431
Longest hedge date	2022
Southern Power
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints	7
Southern Power | Public Utilities, Inventory, Power Position
Derivative [Line Items]
Net Purchased mmBtu | MWh	2
Southern Power | Energy-related, Natural Gas
Derivative [Line Items]
Net Purchased mmBtu	15
Longest hedge date	2020
Alabama Power
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints	4
Alabama Power | Energy-related, Natural Gas
Derivative [Line Items]
Net Purchased mmBtu	74
Longest hedge date	2022
Georgia Power
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints	7
Georgia Power | Energy-related, Natural Gas
Derivative [Line Items]
Net Purchased mmBtu	153
Longest hedge date	2022
Mississippi Power
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints	3
Mississippi Power | Energy-related, Natural Gas
Derivative [Line Items]
Net Purchased mmBtu	63
Longest hedge date	2022
Parent Company and Southern Power
Derivative [Line Items]
Foreign currency cash flow hedge gain (loss) to be reclassified during next 12 months | $	$ (23)