Contingencies - General Litigation Matters (Details) $ in Millions			1 Months Ended			3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 27, 2019 USD ($)	Mar. 14, 2019 plaintiff	Nov. 30, 2018 USD ($) defendant plaintiff	May 31, 2018 USD ($) agreement	Apr. 30, 2018	Jun. 30, 2019 USD ($)	Sep. 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
Compensatory damages sought by litigation	$ 5.0
Punitive damages sought by litigation	$ 50.0
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%