Retirement Benefits (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
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	19,000,000	16,000,000
Interest cost	25,000,000	24,000,000
Expected return on plan assets	(51,000,000)	(49,000,000)
Amortization:
Prior service costs	0	1,000,000
Regulatory asset	0
Net (gain)/loss	14,000,000	10,000,000
Net periodic pension cost (income)	7,000,000	2,000,000
Pension Plans | GEORGIA POWER CO
Pension Plans and Postretirement Plans
Service cost	22,000,000	19,000,000
Interest cost	35,000,000	34,000,000
Expected return on plan assets	(74,000,000)	(71,000,000)
Amortization:
Prior service costs	0	1,000,000
Regulatory asset	0
Net (gain)/loss	17,000,000	14,000,000
Net periodic pension cost (income)	0	(3,000,000)
Pension Plans | GULF POWER CO
Pension Plans and Postretirement Plans
Service cost	4,000,000	3,000,000
Interest cost	5,000,000	5,000,000
Expected return on plan assets	(10,000,000)	(10,000,000)
Amortization:
Prior service costs	0	0
Regulatory asset	0
Net (gain)/loss	2,000,000	2,000,000
Net periodic pension cost (income)	1,000,000	0
Pension Plans | MISSISSIPPI POWER CO
Pension Plans and Postretirement Plans
Service cost	4,000,000	4,000,000
Interest cost	5,000,000	5,000,000
Expected return on plan assets	(10,000,000)	(10,000,000)
Amortization:
Prior service costs	0	0
Regulatory asset	0
Net (gain)/loss	3,000,000	2,000,000
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
Interest cost	1,000,000
Expected return on plan assets	(3,000,000)
Amortization:
Prior service costs	0
Regulatory asset	0
Net (gain)/loss	1,000,000
Net periodic pension cost (income)	1,000,000
Pension Plans | SOUTHERN Co GAS
Pension Plans and Postretirement Plans
Service cost	8,000,000	6,000,000
Interest cost	10,000,000	10,000,000
Expected return on plan assets	(18,000,000)	(18,000,000)
Amortization:
Prior service costs	(1,000,000)	0
Regulatory asset	3,000,000
Net (gain)/loss	3,000,000	5,000,000
Net periodic pension cost (income)	5,000,000	3,000,000
Pension Plans | Southern Company
Pension Plans and Postretirement Plans
Service cost	90,000,000	73,000,000
Interest cost	116,000,000	114,000,000
Expected return on plan assets	(236,000,000)	(224,000,000)
Amortization:
Prior service costs	1,000,000	3,000,000
Regulatory asset	0
Net (gain)/loss	53,000,000	40,000,000
Net periodic pension cost (income)	24,000,000	6,000,000
Postretirement Benefits | ALABAMA POWER CO
Pension Plans and Postretirement Plans
Service cost	1,000,000	1,000,000
Interest cost	4,000,000	5,000,000
Expected return on plan assets	(6,000,000)	(6,000,000)
Amortization:
Prior service costs	1,000,000	1,000,000
Regulatory asset	0
Net (gain)/loss	0	0
Net periodic pension cost (income)	0	1,000,000
Postretirement Benefits | GEORGIA POWER CO
Pension Plans and Postretirement Plans
Service cost	2,000,000	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
Net (gain)/loss	2,000,000	2,000,000
Net periodic pension cost (income)	5,000,000	5,000,000
Postretirement Benefits | GULF 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
Net (gain)/loss	0	0
Net periodic pension cost (income)	1,000,000	1,000,000
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
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
Interest cost	0
Expected return on plan assets	0
Amortization:
Prior service costs	0
Regulatory asset	0
Net (gain)/loss	0
Net periodic pension cost (income)	0
Postretirement Benefits | SOUTHERN Co GAS
Pension Plans and Postretirement Plans
Service cost	1,000,000	1,000,000
Interest cost	2,000,000	3,000,000
Expected return on plan assets	(2,000,000)	(2,000,000)
Amortization:
Prior service costs	0	(1,000,000)
Regulatory asset	1,000,000
Net (gain)/loss	0	1,000,000
Net periodic pension cost (income)	2,000,000	2,000,000
Postretirement Benefits | Southern Company
Pension Plans and Postretirement Plans
Service cost	6,000,000	6,000,000
Interest cost	19,000,000	20,000,000
Expected return on plan assets	(17,000,000)	(16,000,000)
Amortization:
Prior service costs	2,000,000	2,000,000
Regulatory asset	0
Net (gain)/loss	3,000,000	2,000,000
Net periodic pension cost (income)	$ 13,000,000	$ 14,000,000