Contingencies and Regulatory Matters - General (Details) $ in Thousands		3 Months Ended
	Mar. 02, 2018 USD ($) generating_plant	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015
GEORGIA POWER CO
Loss Contingencies [Line Items]
Environmental remediation liability		$ 22,000	$ 22,000
GULF POWER CO
Loss Contingencies [Line Items]
Environmental remediation liability		49,000	52,000
SOUTHERN Co GAS
Loss Contingencies [Line Items]
Environmental remediation liability		369,000	$ 388,000
MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Fuel Cost Recovery				$ 3,000	$ 6,000
RE Roserock, LLC | SOUTHERN POWER CO
Loss Contingencies [Line Items]
Ownership percentage by parent						51.00%
Loss contingency, payments		26,000
Location One | SOUTHERN Co GAS
Loss Contingencies [Line Items]
Environmental remediation liability		$ 2,000
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