Contingencies and Regulatory Matters - General Litigation Matters (Details) $ in Thousands						9 Months Ended
	May 18, 2018 USD ($)	May 04, 2018	Apr. 25, 2018	Mar. 02, 2018 USD ($) generating_plant	Feb. 07, 2018	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
ALABAMA POWER CO
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage						47.00%
MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Public utilities, requested return on equity, percentage					9.33%
MISSISSIPPI POWER CO | MRA Fuel Costs
Loss Contingencies [Line Items]
Over recovered fuel cost						$ 7,000	$ 0
MISSISSIPPI POWER CO | MB Fuel Costs
Loss Contingencies [Line Items]
Over recovered fuel cost						0	0
SOUTHERN Co GAS
Loss Contingencies [Line Items]
Environmental remediation liability						294,000	388,000
GEORGIA POWER CO
Loss Contingencies [Line Items]
Environmental remediation liability						25,000	22,000
GULF POWER CO
Loss Contingencies [Line Items]
Environmental remediation liability						48,000	52,000
Location One | SOUTHERN Co GAS
Loss Contingencies [Line Items]
Environmental remediation liability						$ 2,000
Location One | SOUTHERN Co GAS | Elizabethtown Gas
Loss Contingencies [Line Items]
Environmental remediation liability							$ 85,000
Alabama Department of Environmental Management (ADEM) | ALABAMA POWER CO
Loss Contingencies [Line Items]
Litigation settlement, amount awarded to other party				$ 1,250
Number of electric generating plants | generating_plant				5
Open Access Transmission Tariff
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage						11.25%
RE Roserock, LLC | RE Roserock, LLC | SOUTHERN POWER CO
Loss Contingencies [Line Items]
Ownership percentage by parent						51.00%
Maximum | Open Access Transmission Tariff
Loss Contingencies [Line Items]
Public utilities, requested return on equity, percentage						8.65%
Martin Product Sales, LLC Litigation | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Damages sought by litigation	$ 143,000
Deepwater Horizon Oil Spill Litigation Case | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Litigation settlement awarded from other party						$ 18,000
Nicor Energy Services Company Litigation | SOUTHERN Co GAS
Loss Contingencies [Line Items]
Litigation amount awarded to other party						$ 11,000
Pending Litigation | Shareholder Derivative Lawsuits
Loss Contingencies [Line Items]
Stay period		30 days	30 days