Summary of Significant Accounting Policies - Schedule of Storm Damage Reserves (Details) - Major storms - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Catastrophe Claims [Line Items]
Storm damage reserves	$ 74.0	$ 41.0	$ 40.0
Alabama Power
Liability for Catastrophe Claims [Line Items]
Storm damage reserves	16.0	4.0	3.0
Georgia Power
Liability for Catastrophe Claims [Line Items]
Storm damage reserves	30.0	30.0	30.0
Mississippi Power
Liability for Catastrophe Claims [Line Items]
Storm damage reserves	1.0	3.0	4.0
Gulf Power
Liability for Catastrophe Claims [Line Items]
Storm damage reserves	$ 26.9	$ 3.5	$ 3.5