Contingencies - General Litigation Matters (Details) $ in Millions		1 Months Ended			3 Months Ended	6 Months Ended	12 Months Ended
	Mar. 14, 2019 plaintiff	Nov. 30, 2018 USD ($) defendant plaintiff	May 31, 2018 USD ($) agreement	Apr. 30, 2018	Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	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
Underpayment of refunds		$ 23.5
Martin Product Sales, LLC Litigation | Martin Product Sales, LLC Litigation
Loss Contingencies [Line Items]
Damages sought by litigation			$ 143.0
SOUTHERN POWER CO
Loss Contingencies [Line Items]
Withholding of construction contract payments						$ 26.0
Gain on funds received in excess of initial replacement costs, after tax					$ 12.0
SOUTHERN POWER CO | RE Roserock, LLC | RE Roserock, LLC
Loss Contingencies [Line Items]
Ownership percentage by parent					51.00%	51.00%