Contingencies - Other Matters (Details)		3 Months Ended		9 Months Ended		60 Months Ended
	Oct. 16, 2019 USD ($)	Sep. 30, 2019 USD ($) Facility	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) Facility	Sep. 30, 2018 USD ($)	Jan. 15, 2024 USD ($)	May 17, 2019 USD ($)
Loss Contingencies [Line Items]
Impairment charges		$ 110,000,000	$ 36,000,000	$ 142,000,000	$ 197,000,000
ALABAMA POWER CO | Damage from Unpermitted Discharge of Fluids and/or Pollutants | Plant Gadsden
Loss Contingencies [Line Items]
Possible environmental penalty							$ 250,000
GULF POWER CO | Scenario, Forecast | Plant Daniel Units 1 And 2
Loss Contingencies [Line Items]
Undivided ownership interest to be sold						50.00%
Undivided ownership interest, ownership interest in individual unit						100.00%
MISSISSIPPI POWER CO | Scenario, Forecast | Plant Daniel Units 1 And 2
Loss Contingencies [Line Items]
Payments to acquire business						$ 1
Option to purchase, period						120 days
SOUTHERN Co GAS
Loss Contingencies [Line Items]
Impairment charges		92,000,000	$ 0	92,000,000	$ 42,000,000
SOUTHERN Co GAS | Natural Gas Storage Facility
Loss Contingencies [Line Items]
Impairment charges				92,000,000
Impairment charge, net of tax				65,000,000
Combined net book value		$ 328,000,000		$ 328,000,000
SOUTHERN Co GAS | Natural Gas Storage Facility | California
Loss Contingencies [Line Items]
Number of facilities | Facility		2		2
Subsequent Event | ALABAMA POWER CO
Loss Contingencies [Line Items]
Civil penalties	$ 50,000
Fish restocking costs	$ 172,000