Retirement Benefits (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
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 | ALABAMA POWER CO
Pension Plans and Postretirement Plans
Service cost	17,000,000	19,000,000
Interest cost	28,000,000	25,000,000
Expected return on plan assets	(51,000,000)	(51,000,000)
Amortization:
Prior service costs	0	0
Regulatory asset	0	0
Net (gain)/loss	9,000,000	14,000,000
Net periodic pension cost (income)	3,000,000	7,000,000
Pension Plans | GEORGIA POWER CO
Pension Plans and Postretirement Plans
Service cost	19,000,000	22,000,000
Interest cost	39,000,000	35,000,000
Expected return on plan assets	(73,000,000)	(74,000,000)
Amortization:
Prior service costs	0	0
Regulatory asset	0	0
Net (gain)/loss	11,000,000	17,000,000
Net periodic pension cost (income)	(4,000,000)	0
Pension Plans | MISSISSIPPI POWER CO
Pension Plans and Postretirement Plans
Service cost	3,000,000	4,000,000
Interest cost	6,000,000	5,000,000
Expected return on plan assets	(10,000,000)	(10,000,000)
Amortization:
Prior service costs	0	0
Regulatory asset	0	0
Net (gain)/loss	1,000,000	3,000,000
Net periodic pension cost (income)	0	2,000,000
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	(2,000,000)	(3,000,000)
Amortization:
Prior service costs	0	0
Regulatory asset	0	0
Net (gain)/loss	0	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	6,000,000	8,000,000
Interest cost	9,000,000	10,000,000
Expected return on plan assets	(15,000,000)	(18,000,000)
Amortization:
Prior service costs	(1,000,000)	(1,000,000)
Regulatory asset	3,000,000	3,000,000
Net (gain)/loss	1,000,000	3,000,000
Net periodic pension cost (income)	3,000,000	5,000,000
Pension Plans | Southern Company
Pension Plans and Postretirement Plans
Service cost	73,000,000	90,000,000
Interest cost	123,000,000	116,000,000
Expected return on plan assets	(221,000,000)	(236,000,000)
Amortization:
Prior service costs	0	1,000,000
Regulatory asset	0	0
Net (gain)/loss	30,000,000	53,000,000
Net periodic pension cost (income)	5,000,000	24,000,000
Postretirement Benefits | ALABAMA POWER CO
Pension Plans and Postretirement Plans
Service cost	1,000,000	1,000,000
Interest cost	4,000,000	4,000,000
Expected return on plan assets	(6,000,000)	(6,000,000)
Amortization:
Prior service costs	1,000,000	1,000,000
Regulatory asset	0	0
Net (gain)/loss	0	0
Net periodic pension cost (income)	0	0
Postretirement Benefits | GEORGIA POWER CO
Pension Plans and Postretirement Plans
Service cost	1,000,000	2,000,000
Interest cost	7,000,000	7,000,000
Expected return on plan assets	(6,000,000)	(6,000,000)
Amortization:
Prior service costs	0	0
Regulatory asset	0	0
Net (gain)/loss	0	2,000,000
Net periodic pension cost (income)	2,000,000	5,000,000
Postretirement Benefits | MISSISSIPPI POWER CO
Pension Plans and Postretirement Plans
Service cost	0	0
Interest cost	1,000,000	1,000,000
Expected return on plan assets	0	0
Amortization:
Prior service costs	0	0
Regulatory asset	0	0
Net (gain)/loss	0	0
Net periodic pension cost (income)	1,000,000	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	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	1,000,000	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	0
Regulatory asset	2,000,000	1,000,000
Net (gain)/loss	(1,000,000)	0
Net periodic pension cost (income)	2,000,000	2,000,000
Postretirement Benefits | Southern Company
Pension Plans and Postretirement Plans
Service cost	5,000,000	6,000,000
Interest cost	17,000,000	19,000,000
Expected return on plan assets	(16,000,000)	(17,000,000)
Amortization:
Prior service costs	1,000,000	2,000,000
Regulatory asset	0	0
Net (gain)/loss	(1,000,000)	3,000,000
Net periodic pension cost (income)	$ 6,000,000	$ 13,000,000