Retirement Benefits (Details) - USD ($)	3 Months Ended	6 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected future employer contributions, remainder of fiscal year	$ 0			$ 0
Pension Plans \| Southern Company
Pension Plans and Postretirement Plans
Service cost	73,000,000	$ 68,000,000		220,000,000	$ 192,000,000
Interest cost	114,000,000	110,000,000		341,000,000	311,000,000
Expected return on plan assets	(224,000,000)	(203,000,000)		(673,000,000)	(577,000,000)
Amortization:
Prior service costs	3,000,000	3,000,000		9,000,000	10,000,000
Net (gain)/loss	41,000,000	45,000,000		122,000,000	120,000,000
Net periodic pension cost (income)	7,000,000	23,000,000		19,000,000	56,000,000
Pension Plans \| SOUTHERN Co GAS
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	6,000,000	7,000,000		17,000,000
Interest cost	10,000,000	10,000,000		30,000,000
Expected return on plan assets	(18,000,000)	(17,000,000)		(53,000,000)
Amortization of regulatory asset		6,000,000
Amortization:
Prior service costs				(1,000,000)
Net (gain)/loss	5,000,000			15,000,000
Net periodic pension cost (income)	3,000,000	6,000,000		8,000,000
Pension Plans \| ALABAMA POWER CO
Pension Plans and Postretirement Plans
Service cost	15,000,000	14,000,000		47,000,000	43,000,000
Interest cost	25,000,000	23,000,000		73,000,000	71,000,000
Expected return on plan assets	(49,000,000)	(46,000,000)		(147,000,000)	(138,000,000)
Amortization:
Prior service costs	1,000,000	1,000,000		2,000,000	2,000,000
Net (gain)/loss	10,000,000	10,000,000		31,000,000	30,000,000
Net periodic pension cost (income)	2,000,000	2,000,000		6,000,000	8,000,000
Pension Plans \| GEORGIA POWER CO
Pension Plans and Postretirement Plans
Service cost	19,000,000	17,000,000		56,000,000	52,000,000
Interest cost	34,000,000	34,000,000		103,000,000	102,000,000
Expected return on plan assets	(71,000,000)	(64,000,000)		(212,000,000)	(193,000,000)
Amortization:
Prior service costs	0	1,000,000		2,000,000	4,000,000
Net (gain)/loss	15,000,000	14,000,000		43,000,000	41,000,000
Net periodic pension cost (income)	(3,000,000)	2,000,000		(8,000,000)	6,000,000
Pension Plans \| GULF POWER CO
Pension Plans and Postretirement Plans
Service cost	3,000,000	3,000,000		10,000,000	9,000,000
Interest cost	5,000,000	5,000,000		15,000,000	14,000,000
Expected return on plan assets	(10,000,000)	(9,000,000)		(29,000,000)	(26,000,000)
Amortization:
Prior service costs	0	0		0	1,000,000
Net (gain)/loss	2,000,000	2,000,000		5,000,000	5,000,000
Net periodic pension cost (income)	0	1,000,000		1,000,000	3,000,000
Pension Plans \| MISSISSIPPI POWER CO
Pension Plans and Postretirement Plans
Service cost	4,000,000	3,000,000		11,000,000	9,000,000
Interest cost	5,000,000	4,000,000		15,000,000	14,000,000
Expected return on plan assets	(9,000,000)	(9,000,000)		(29,000,000)	(26,000,000)
Amortization:
Prior service costs	0	1,000,000		1,000,000	1,000,000
Net (gain)/loss	1,000,000	2,000,000		5,000,000	5,000,000
Net periodic pension cost (income)	1,000,000	1,000,000		3,000,000	3,000,000
Postretirement Benefits \| Southern Company
Pension Plans and Postretirement Plans
Service cost	6,000,000	6,000,000		18,000,000	17,000,000
Interest cost	19,000,000	20,000,000		59,000,000	55,000,000
Expected return on plan assets	(16,000,000)	(16,000,000)		(49,000,000)	(44,000,000)
Amortization:
Prior service costs	2,000,000	1,000,000		5,000,000	4,000,000
Net (gain)/loss	3,000,000	5,000,000		10,000,000	12,000,000
Net periodic pension cost (income)	14,000,000	16,000,000		43,000,000	44,000,000
Postretirement Benefits \| SOUTHERN Co GAS
Pension Plans and Postretirement Plans
Service cost	1,000,000	1,000,000		2,000,000
Interest cost	3,000,000	2,000,000		8,000,000
Expected return on plan assets	(2,000,000)	(2,000,000)		(5,000,000)
Amortization of regulatory asset		1,000,000
Amortization:
Prior service costs	(1,000,000)			(2,000,000)
Net (gain)/loss	1,000,000			3,000,000
Net periodic pension cost (income)	2,000,000	2,000,000		6,000,000
Postretirement Benefits \| ALABAMA POWER CO
Pension Plans and Postretirement Plans
Service cost	1,000,000	1,000,000		4,000,000	4,000,000
Interest cost	4,000,000	5,000,000		13,000,000	14,000,000
Expected return on plan assets	(5,000,000)	(6,000,000)		(19,000,000)	(19,000,000)
Amortization:
Prior service costs	1,000,000	1,000,000		3,000,000	3,000,000
Net (gain)/loss	0	0		1,000,000	1,000,000
Net periodic pension cost (income)	1,000,000	1,000,000		2,000,000	3,000,000
Postretirement Benefits \| GEORGIA POWER CO
Pension Plans and Postretirement Plans
Service cost	2,000,000	2,000,000		5,000,000	5,000,000
Interest cost	6,000,000	7,000,000		21,000,000	22,000,000
Expected return on plan assets	(6,000,000)	(6,000,000)		(18,000,000)	(17,000,000)
Amortization:
Prior service costs	0	0		1,000,000	1,000,000
Net (gain)/loss	3,000,000	3,000,000		6,000,000	7,000,000
Net periodic pension cost (income)	5,000,000	6,000,000		15,000,000	18,000,000
Postretirement Benefits \| GULF POWER CO
Pension Plans and Postretirement Plans
Service cost	0	0		1,000,000	1,000,000
Interest cost	1,000,000	1,000,000		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
Net (gain)/loss	0	0		0	0
Net periodic pension cost (income)	1,000,000	1,000,000		2,000,000	2,000,000
Postretirement Benefits \| MISSISSIPPI POWER CO
Pension Plans and Postretirement Plans
Service cost	0	0		1,000,000	1,000,000
Interest cost	1,000,000	0		3,000,000	2,000,000
Expected return on plan assets	0	0		(1,000,000)	(1,000,000)
Amortization:
Prior service costs	0	0		0	0
Net (gain)/loss	0	1,000,000		0	1,000,000
Net periodic pension cost (income)	$ 1,000,000	$ 1,000,000		$ 3,000,000	$ 3,000,000
Predecessor \| Pension Plans \| SOUTHERN Co GAS
Pension Plans and Postretirement Plans
Service cost			$ 13,000,000
Interest cost			21,000,000
Expected return on plan assets			(33,000,000)
Amortization:
Prior service costs			(1,000,000)
Net (gain)/loss			13,000,000
Net periodic pension cost (income)			13,000,000
Predecessor \| Postretirement Benefits \| SOUTHERN Co GAS
Pension Plans and Postretirement Plans
Service cost			1,000,000
Interest cost			5,000,000
Expected return on plan assets			(3,000,000)
Amortization:
Prior service costs			(1,000,000)
Net (gain)/loss			2,000,000
Net periodic pension cost (income)			$ 4,000,000

Defined Benefit Plan, Net Periodic Benefit Cost, Amortization [Abstract]

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Amount of gain (loss) recognized in net periodic benefit (cost) credit of defined benefit plan.

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Amount of contributions expected to be received by defined benefit plan from employer in fiscal year. Excludes contributions paid by employer in current fiscal year.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2709-114920

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Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan.

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Amount of cost recognized for passage of time related to defined benefit plan.

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Amount of net periodic benefit cost (credit) for defined benefit plan.

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Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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-Name Accounting Standards Codification
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