RETIREMENT BENEFITS (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Amortization:
Net (gain)/loss
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected future employer contributions, remainder of fiscal year	$ 0		$ 0
Pension Plans and Postretirement Plans
Service cost	94,000,000	73,000,000	282,000,000	$ 219,000,000
Interest cost	108,000,000	123,000,000	324,000,000	369,000,000
Expected return on plan assets	(274,000,000)	(222,000,000)	(824,000,000)	(664,000,000)
Amortization:
Prior service costs	0	1,000,000	1,000,000	2,000,000
Regulatory asset	0	0	0	0
Net (gain)/loss	67,000,000	30,000,000	201,000,000	90,000,000
Net periodic pension cost (income)	(5,000,000)	5,000,000	(16,000,000)	16,000,000
Pension Plans | ALABAMA POWER CO
Pension Plans and Postretirement Plans
Service cost	23,000,000	17,000,000	67,000,000	51,000,000
Interest cost	25,000,000	28,000,000	75,000,000	85,000,000
Expected return on plan assets	(66,000,000)	(51,000,000)	(198,000,000)	(154,000,000)
Amortization:
Prior service costs	1,000,000	0	1,000,000	1,000,000
Regulatory asset	0	0	0	0
Net (gain)/loss	17,000,000	9,000,000	53,000,000	27,000,000
Net periodic pension cost (income)	0	3,000,000	(2,000,000)	10,000,000
Pension Plans | GEORGIA POWER CO
Pension Plans and Postretirement Plans
Service cost	24,000,000	19,000,000	72,000,000	56,000,000
Interest cost	33,000,000	38,000,000	100,000,000	116,000,000
Expected return on plan assets	(87,000,000)	(73,000,000)	(261,000,000)	(219,000,000)
Amortization:
Prior service costs	0	0	1,000,000	1,000,000
Regulatory asset	0	0	0	0
Net (gain)/loss	22,000,000	11,000,000	65,000,000	33,000,000
Net periodic pension cost (income)	(8,000,000)	(5,000,000)	(23,000,000)	(13,000,000)
Pension Plans | MISSISSIPPI POWER CO
Pension Plans and Postretirement Plans
Service cost	3,000,000	3,000,000	11,000,000	9,000,000
Interest cost	5,000,000	6,000,000	15,000,000	17,000,000
Expected return on plan assets	(13,000,000)	(10,000,000)	(38,000,000)	(30,000,000)
Amortization:
Prior service costs	0	0	0	0
Regulatory asset	0	0	0	0
Net (gain)/loss	4,000,000	1,000,000	10,000,000	4,000,000
Net periodic pension cost (income)	(1,000,000)	0	(2,000,000)	0
Pension Plans | SOUTHERN POWER CO
Pension Plans and Postretirement Plans
Service cost	2,000,000	2,000,000	6,000,000	5,000,000
Interest cost	1,000,000	1,000,000	4,000,000	4,000,000
Expected return on plan assets	(4,000,000)	(2,000,000)	(10,000,000)	(7,000,000)
Amortization:
Prior service costs	0	0	0	0
Regulatory asset	0	0	0	0
Net (gain)/loss	1,000,000	0	2,000,000	0
Net periodic pension cost (income)	0	1,000,000	2,000,000	2,000,000
Pension Plans | SOUTHERN Co GAS
Pension Plans and Postretirement Plans
Service cost	8,000,000	7,000,000	24,000,000	19,000,000
Interest cost	8,000,000	9,000,000	23,000,000	27,000,000
Expected return on plan assets	(20,000,000)	(15,000,000)	(59,000,000)	(45,000,000)
Amortization:
Prior service costs	(1,000,000)	(1,000,000)	(2,000,000)	(2,000,000)
Regulatory asset	4,000,000	3,000,000	12,000,000	10,000,000
Net (gain)/loss	2,000,000	1,000,000	7,000,000	2,000,000
Net periodic pension cost (income)	1,000,000	4,000,000	5,000,000	11,000,000
Postretirement Benefits
Pension Plans and Postretirement Plans
Service cost	6,000,000	5,000,000	17,000,000	14,000,000
Interest cost	13,000,000	18,000,000	40,000,000	52,000,000
Expected return on plan assets	(18,000,000)	(16,000,000)	(54,000,000)	(49,000,000)
Amortization:
Prior service costs	0	0	(1,000,000)	2,000,000
Regulatory asset	0	0	0	0
Net (gain)/loss	1,000,000	(1,000,000)	2,000,000	(2,000,000)
Net periodic pension cost (income)	2,000,000	6,000,000	4,000,000	17,000,000
Postretirement Benefits | ALABAMA POWER CO
Pension Plans and Postretirement Plans
Service cost	1,000,000	1,000,000	4,000,000	3,000,000
Interest cost	4,000,000	4,000,000	10,000,000	12,000,000
Expected return on plan assets	(7,000,000)	(6,000,000)	(21,000,000)	(19,000,000)
Amortization:
Prior service costs	0	1,000,000	0	3,000,000
Regulatory asset	0	0	0	0
Net (gain)/loss	0	0	0	0
Net periodic pension cost (income)	(2,000,000)	0	(7,000,000)	(1,000,000)
Postretirement Benefits | GEORGIA POWER CO
Pension Plans and Postretirement Plans
Service cost	2,000,000	2,000,000	5,000,000	4,000,000
Interest cost	5,000,000	6,000,000	15,000,000	19,000,000
Expected return on plan assets	(7,000,000)	(7,000,000)	(20,000,000)	(19,000,000)
Amortization:
Prior service costs	(1,000,000)	0	(1,000,000)	0
Regulatory asset	0	0	0	0
Net (gain)/loss	1,000,000	1,000,000	2,000,000	1,000,000
Net periodic pension cost (income)	0	2,000,000	1,000,000	5,000,000
Postretirement Benefits | MISSISSIPPI POWER CO
Pension Plans and Postretirement Plans
Service cost	(1,000,000)	1,000,000	0	1,000,000
Interest cost	1,000,000	0	2,000,000	2,000,000
Expected return on plan assets	0	0	(1,000,000)	(1,000,000)
Amortization:
Prior service costs	0	0	0	0
Regulatory asset	0	0	0	0
Net (gain)/loss	0	0	0	0
Net periodic pension cost (income)	0	1,000,000	1,000,000	2,000,000
Postretirement Benefits | SOUTHERN POWER CO
Pension Plans and Postretirement Plans
Service cost	1,000,000	0	1,000,000	0
Interest cost	0	0	0	0
Expected return on plan assets	0	0	0	0
Amortization:
Prior service costs	0	0	0	0
Regulatory asset	0	0	0	0
Net (gain)/loss	0	0	0	0
Net periodic pension cost (income)	1,000,000	0	1,000,000	0
Postretirement Benefits | SOUTHERN Co GAS
Pension Plans and Postretirement Plans
Service cost	0	0	1,000,000	1,000,000
Interest cost	2,000,000	2,000,000	5,000,000	7,000,000
Expected return on plan assets	(2,000,000)	(1,000,000)	(5,000,000)	(4,000,000)
Amortization:
Prior service costs	0	0	0	0
Regulatory asset	2,000,000	1,000,000	5,000,000	4,000,000
Net (gain)/loss	(1,000,000)	0	(2,000,000)	(2,000,000)
Net periodic pension cost (income)	$ 1,000,000	$ 2,000,000	$ 4,000,000	$ 6,000,000