Derivatives - Energy-Related Derivative Contracts (Details) - Energy-related, Natural Gas MMBTU in Millions	12 Months Ended
	Dec. 31, 2018 MMBTU
Derivative [Line Items]
Net Purchased mmBtu	431
Longest Hedge Date	2022
Longest Non-Hedge Date	2029
Alabama Power
Derivative [Line Items]
Net Purchased mmBtu	74
Longest Hedge Date	2022
Georgia Power
Derivative [Line Items]
Net Purchased mmBtu	153
Longest Hedge Date	2022
Mississippi Power
Derivative [Line Items]
Net Purchased mmBtu	63
Longest Hedge Date	2022
Southern Power
Derivative [Line Items]
Net Purchased mmBtu	15
Longest Hedge Date	2020
Southern Company Gas
Derivative [Line Items]
Net Purchased mmBtu	120
Longest Hedge Date	2021
Longest Non-Hedge Date	2029
Long | Not Designated as Hedging Instrument | Southern Company Gas
Derivative [Line Items]
Derivative nonmonetary notional amount net long short position volume	4,159
Short | Not Designated as Hedging Instrument | Southern Company Gas
Derivative [Line Items]
Derivative nonmonetary notional amount net long short position volume	4,039