Contingencies - General Litigation Matters (Details) $ in Millions	1 Months Ended				3 Months Ended	12 Months Ended
	May 31, 2019 USD ($)	Nov. 30, 2018 defendant plaintiff	May 31, 2018 USD ($) agreement	Apr. 30, 2018	Mar. 31, 2019 USD ($)	Dec. 31, 2017 claim
Loss Contingencies [Line Items]
Number of plaintiffs | plaintiff		10
Pending Litigation | Putative Securities Class Action
Loss Contingencies [Line Items]
Increase in the purported class period						1 day
Pending Litigation | Shareholder Derivative Lawsuits
Loss Contingencies [Line Items]
Pending number of claims | claim						2
Stay period			30 days	30 days
MISSISSIPPI POWER CO | Pending Litigation | Martin Product Sales, LLC Litigation
Loss Contingencies [Line Items]
Number of agreements | agreement			2
MISSISSIPPI POWER CO | Pending Litigation | Purported Violations Of Mississippi Consumer Protection Act
Loss Contingencies [Line Items]
Number of plaintiffs | plaintiff		4
Number of defendants | defendant		3
Martin Product Sales, LLC Litigation | Martin Product Sales, LLC Litigation
Loss Contingencies [Line Items]
Damages sought by litigation			$ 143
SOUTHERN POWER CO
Loss Contingencies [Line Items]
Withholding of construction contract payments					$ 26
SOUTHERN POWER CO | RE Roserock, LLC | RE Roserock, LLC
Loss Contingencies [Line Items]
Ownership percentage by parent					51.00%
RE Roserock, LLC
Loss Contingencies [Line Items]
Insurance proceeds					$ 5
RE Roserock, LLC | Scenario, Forecast | Subsequent Event
Loss Contingencies [Line Items]
Amount paid to other party	$ 26