RETIREMENT BENEFITS (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 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
Pension Plans and Postretirement Plans
Service cost	94,000,000	73,000,000
Interest cost	108,000,000	123,000,000
Expected return on plan assets	(275,000,000)	(221,000,000)
Amortization:
Prior service costs	1,000,000	0
Regulatory asset	0	0
Net (gain)/loss	67,000,000	30,000,000
Net periodic pension cost (income)	(5,000,000)	5,000,000
Pension Plans | ALABAMA POWER CO
Pension Plans and Postretirement Plans
Service cost	22,000,000	17,000,000
Interest cost	25,000,000	28,000,000
Expected return on plan assets	(66,000,000)	(51,000,000)
Amortization:
Prior service costs	0	0
Regulatory asset	0	0
Net (gain)/loss	18,000,000	9,000,000
Net periodic pension cost (income)	(1,000,000)	3,000,000
Pension Plans | GEORGIA POWER CO
Pension Plans and Postretirement Plans
Service cost	24,000,000	19,000,000
Interest cost	33,000,000	39,000,000
Expected return on plan assets	(87,000,000)	(73,000,000)
Amortization:
Prior service costs	0	0
Regulatory asset	0	0
Net (gain)/loss	22,000,000	11,000,000
Net periodic pension cost (income)	(8,000,000)	(4,000,000)
Pension Plans | MISSISSIPPI POWER CO
Pension Plans and Postretirement Plans
Service cost	4,000,000	3,000,000
Interest cost	5,000,000	6,000,000
Expected return on plan assets	(13,000,000)	(10,000,000)
Amortization:
Prior service costs	0	0
Regulatory asset	0	0
Net (gain)/loss	3,000,000	1,000,000
Net periodic pension cost (income)	(1,000,000)	0
Pension Plans | SOUTHERN POWER CO
Pension Plans and Postretirement Plans
Service cost	2,000,000	2,000,000
Interest cost	1,000,000	1,000,000
Expected return on plan assets	(3,000,000)	(2,000,000)
Amortization:
Prior service costs	0	0
Regulatory asset	0	0
Net (gain)/loss	1,000,000	0
Net periodic pension cost (income)	1,000,000	1,000,000
Pension Plans | SOUTHERN Co GAS
Pension Plans and Postretirement Plans
Service cost	8,000,000	6,000,000
Interest cost	8,000,000	9,000,000
Expected return on plan assets	(19,000,000)	(15,000,000)
Amortization:
Prior service costs	(1,000,000)	(1,000,000)
Regulatory asset	4,000,000	3,000,000
Net (gain)/loss	2,000,000	1,000,000
Net periodic pension cost (income)	2,000,000	3,000,000
Postretirement Benefits
Pension Plans and Postretirement Plans
Service cost	5,000,000	5,000,000
Interest cost	13,000,000	17,000,000
Expected return on plan assets	(18,000,000)	(16,000,000)
Amortization:
Prior service costs		1,000,000
Regulatory asset	0	0
Net (gain)/loss	1,000,000	(1,000,000)
Net periodic pension cost (income)	1,000,000	6,000,000
Postretirement Benefits | ALABAMA POWER CO
Pension Plans and Postretirement Plans
Service cost	2,000,000	1,000,000
Interest cost	3,000,000	4,000,000
Expected return on plan assets	(7,000,000)	(6,000,000)
Amortization:
Prior service costs		1,000,000
Regulatory asset	0	0
Net (gain)/loss	0	0
Net periodic pension cost (income)	(2,000,000)	0
Postretirement Benefits | GEORGIA POWER CO
Pension Plans and Postretirement Plans
Service cost	1,000,000	1,000,000
Interest cost	5,000,000	7,000,000
Expected return on plan assets	(7,000,000)	(6,000,000)
Amortization:
Prior service costs		0
Regulatory asset	0	0
Net (gain)/loss	1,000,000	0
Net periodic pension cost (income)	0	2,000,000
Postretirement Benefits | MISSISSIPPI POWER CO
Pension Plans and Postretirement Plans
Service cost	0	0
Interest cost	0	1,000,000
Expected return on plan assets	0	0
Amortization:
Prior service costs		0
Regulatory asset	0	0
Net (gain)/loss	0	0
Net periodic pension cost (income)	0	1,000,000
Postretirement Benefits | SOUTHERN POWER CO
Pension Plans and Postretirement Plans
Service cost	0	0
Interest cost	0	0
Expected return on plan assets	0	0
Amortization:
Prior service costs		0
Regulatory asset	0	0
Net (gain)/loss	0	0
Net periodic pension cost (income)	0	0
Postretirement Benefits | SOUTHERN Co GAS
Pension Plans and Postretirement Plans
Service cost	0	1,000,000
Interest cost	2,000,000	2,000,000
Expected return on plan assets	(2,000,000)	(2,000,000)
Amortization:
Prior service costs		0
Regulatory asset	2,000,000	2,000,000
Net (gain)/loss	(1,000,000)	(1,000,000)
Net periodic pension cost (income)	$ 1,000,000	$ 2,000,000