CONTINGENCIES - General Litigation Matters (Details) $ in Millions	1 Months Ended				3 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($)	Mar. 31, 2019 plaintiff	Nov. 30, 2018 USD ($) defendant plaintiff	May 31, 2018 USD ($) agreement	Sep. 30, 2020 USD ($)	Dec. 31, 2018	Dec. 31, 2017 claim
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
MISSISSIPPI POWER CO | Pending Litigation | Purported Violations Of Mississippi Consumer Protection Act
Loss Contingencies [Line Items]
Number of plaintiffs | plaintiff		4	10
Number of defendants | defendant			3
Underpayment of refunds			$ 23.5
MISSISSIPPI POWER CO | Settled Litigation | Martin Product Sales, LLC Litigation
Loss Contingencies [Line Items]
Number of agreements | agreement				2
Damages sought by litigation				$ 143.0	$ 76.0
Compensatory damages sought by litigation	$ 5.0
Punitive damages sought by litigation	$ 50.0