Retirement Benefits (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
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	109,000,000	$ 94,000,000
Interest cost	87,000,000	108,000,000
Expected return on plan assets	(298,000,000)	(275,000,000)
Amortization:
Prior service costs	0	1,000,000
Regulatory asset	0	0
Net (gain)/loss	78,000,000	67,000,000
Net periodic pension cost (income)	(24,000,000)	(5,000,000)
Pension Plans | ALABAMA POWER CO
Pension Plans and Postretirement Plans
Service cost	26,000,000	22,000,000
Interest cost	20,000,000	25,000,000
Expected return on plan assets	(72,000,000)	(66,000,000)
Amortization:
Prior service costs	0	0
Regulatory asset	0	0
Net (gain)/loss	21,000,000	18,000,000
Net periodic pension cost (income)	(5,000,000)	(1,000,000)
Pension Plans | GEORGIA POWER CO
Pension Plans and Postretirement Plans
Service cost	28,000,000	24,000,000
Interest cost	26,000,000	33,000,000
Expected return on plan assets	(94,000,000)	(87,000,000)
Amortization:
Prior service costs	0	0
Regulatory asset	0	0
Net (gain)/loss	25,000,000	22,000,000
Net periodic pension cost (income)	(15,000,000)	(8,000,000)
Pension Plans | MISSISSIPPI POWER CO
Pension Plans and Postretirement Plans
Service cost	4,000,000	4,000,000
Interest cost	4,000,000	5,000,000
Expected return on plan assets	(14,000,000)	(13,000,000)
Amortization:
Prior service costs	0	0
Regulatory asset	0	0
Net (gain)/loss	4,000,000	3,000,000
Net periodic pension cost (income)	(2,000,000)	(1,000,000)
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)	(3,000,000)
Amortization:
Prior service costs	0	0
Regulatory asset	0	0
Net (gain)/loss	1,000,000	1,000,000
Net periodic pension cost (income)	1,000,000	1,000,000
Pension Plans | SOUTHERN Co GAS
Pension Plans and Postretirement Plans
Service cost	9,000,000	8,000,000
Interest cost	6,000,000	8,000,000
Expected return on plan assets	(21,000,000)	(19,000,000)
Amortization:
Prior service costs	(1,000,000)	(1,000,000)
Regulatory asset	4,000,000	4,000,000
Net (gain)/loss	3,000,000	2,000,000
Net periodic pension cost (income)	0	2,000,000
Postretirement Benefits
Pension Plans and Postretirement Plans
Service cost	6,000,000	5,000,000
Interest cost	8,000,000	13,000,000
Expected return on plan assets	(19,000,000)	(18,000,000)
Amortization:
Regulatory asset	0	0
Net (gain)/loss	1,000,000	1,000,000
Net periodic pension cost (income)	(4,000,000)	1,000,000
Postretirement Benefits | ALABAMA POWER CO
Pension Plans and Postretirement Plans
Service cost	1,000,000	2,000,000
Interest cost	2,000,000	3,000,000
Expected return on plan assets	(7,000,000)	(7,000,000)
Amortization:
Regulatory asset	0	0
Net (gain)/loss	0	0
Net periodic pension cost (income)	(4,000,000)	(2,000,000)
Postretirement Benefits | GEORGIA POWER CO
Pension Plans and Postretirement Plans
Service cost	2,000,000	1,000,000
Interest cost	3,000,000	5,000,000
Expected return on plan assets	(7,000,000)	(7,000,000)
Amortization:
Regulatory asset	0	0
Net (gain)/loss	1,000,000	1,000,000
Net periodic pension cost (income)	(1,000,000)	0
Postretirement Benefits | MISSISSIPPI POWER CO
Pension Plans and Postretirement Plans
Service cost	0	0
Interest cost	0	0
Expected return on plan assets	0	0
Amortization:
Regulatory asset	0	0
Net (gain)/loss	0	0
Net periodic pension cost (income)	0	0
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:
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	0
Interest cost	1,000,000	2,000,000
Expected return on plan assets	(2,000,000)	(2,000,000)
Amortization:
Regulatory asset	2,000,000	2,000,000
Net (gain)/loss	(1,000,000)	(1,000,000)
Net periodic pension cost (income)	$ 0	$ 1,000,000