PROPERTY, PLANT, AND EQUIPMENT - Assets Subject to Lien (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Assets, primarily related to property, plant, and equipment, subject to lien	$ 118,700	$ 116,914	$ 111,005
Mississippi Power
Property, Plant and Equipment [Line Items]
Assets, primarily related to property, plant, and equipment, subject to lien	5,035	4,886
Mississippi Power | Chevron
Property, Plant and Equipment [Line Items]
Net book value of co-generation assets	118
Southern Power
Property, Plant and Equipment [Line Items]
Assets, primarily related to property, plant, and equipment, subject to lien	14,300	$ 14,883
Southern Power | Senior Lien
Property, Plant and Equipment [Line Items]
Assets, primarily related to property, plant, and equipment, subject to lien	$ 547