Retirement Benefits (Details) - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 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		$ 0
Pension Plans \| ALABAMA POWER CO
Pension Plans and Postretirement Plans
Service cost	20,000,000	16,000,000	39,000,000	$ 32,000,000
Interest cost	25,000,000	24,000,000	50,000,000	48,000,000
Expected return on plan assets	(53,000,000)	(49,000,000)	(104,000,000)	(98,000,000)
Amortization:
Prior service costs	1,000,000	0	1,000,000	1,000,000
Regulatory asset	0		0
Net (gain)/loss	13,000,000	11,000,000	27,000,000	21,000,000
Net periodic pension cost (income)	6,000,000	2,000,000	13,000,000	4,000,000
Pension Plans \| GEORGIA POWER CO
Pension Plans and Postretirement Plans
Service cost	21,000,000	18,000,000	43,000,000	37,000,000
Interest cost	35,000,000	35,000,000	70,000,000	69,000,000
Expected return on plan assets	(74,000,000)	(70,000,000)	(148,000,000)	(141,000,000)
Amortization:
Prior service costs	1,000,000	1,000,000	1,000,000	2,000,000
Regulatory asset	0		0
Net (gain)/loss	17,000,000	14,000,000	34,000,000	28,000,000
Net periodic pension cost (income)	0	(2,000,000)	0	(5,000,000)
Pension Plans \| GULF POWER CO
Pension Plans and Postretirement Plans
Service cost	4,000,000	4,000,000	8,000,000	7,000,000
Interest cost	5,000,000	5,000,000	10,000,000	10,000,000
Expected return on plan assets	(10,000,000)	(9,000,000)	(20,000,000)	(19,000,000)
Amortization:
Prior service costs	0	0	0	0
Regulatory asset	0		0
Net (gain)/loss	3,000,000	1,000,000	5,000,000	3,000,000
Net periodic pension cost (income)	2,000,000	1,000,000	3,000,000	1,000,000
Pension Plans \| MISSISSIPPI POWER CO
Pension Plans and Postretirement Plans
Service cost	4,000,000	3,000,000	8,000,000	7,000,000
Interest cost	5,000,000	5,000,000	10,000,000	10,000,000
Expected return on plan assets	(10,000,000)	(10,000,000)	(20,000,000)	(20,000,000)
Amortization:
Prior service costs	0	1,000,000	0	1,000,000
Regulatory asset	0		0
Net (gain)/loss	2,000,000	2,000,000	5,000,000	4,000,000
Net periodic pension cost (income)	1,000,000	1,000,000	3,000,000	2,000,000
Pension Plans \| SOUTHERN POWER CO
Pension Plans and Postretirement Plans
Service cost	2,000,000		4,000,000
Interest cost	2,000,000		3,000,000
Expected return on plan assets	(2,000,000)		(5,000,000)
Amortization:
Prior service costs	0		0
Regulatory asset	0		0
Net (gain)/loss	0		1,000,000
Net periodic pension cost (income)	2,000,000		3,000,000
Pension Plans \| SOUTHERN Co GAS
Pension Plans and Postretirement Plans
Service cost	8,000,000	5,000,000	16,000,000	11,000,000
Interest cost	9,000,000	10,000,000	19,000,000	20,000,000
Expected return on plan assets	(17,000,000)	(17,000,000)	(35,000,000)	(35,000,000)
Amortization:
Prior service costs	0	(1,000,000)	(1,000,000)	(1,000,000)
Regulatory asset	4,000,000		7,000,000
Net (gain)/loss	3,000,000	5,000,000	6,000,000	10,000,000
Net periodic pension cost (income)	7,000,000	2,000,000	12,000,000	5,000,000
Pension Plans \| Southern Company
Pension Plans and Postretirement Plans
Service cost	89,000,000	74,000,000	179,000,000	147,000,000
Interest cost	116,000,000	113,000,000	232,000,000	227,000,000
Expected return on plan assets	(235,000,000)	(225,000,000)	(471,000,000)	(449,000,000)
Amortization:
Prior service costs	1,000,000	3,000,000	2,000,000	6,000,000
Regulatory asset	0		0
Net (gain)/loss	54,000,000	41,000,000	107,000,000	81,000,000
Net periodic pension cost (income)	25,000,000	6,000,000	49,000,000	12,000,000
Postretirement Benefits \| ALABAMA POWER CO
Pension Plans and Postretirement Plans
Service cost	2,000,000	2,000,000	3,000,000	3,000,000
Interest cost	4,000,000	4,000,000	8,000,000	9,000,000
Expected return on plan assets	(7,000,000)	(8,000,000)	(13,000,000)	(14,000,000)
Amortization:
Prior service costs	1,000,000	1,000,000		2,000,000
Regulatory asset	0		2,000,000
Net (gain)/loss	1,000,000	1,000,000	1,000,000	1,000,000
Net periodic pension cost (income)	1,000,000	0	1,000,000	1,000,000
Postretirement Benefits \| GEORGIA POWER CO
Pension Plans and Postretirement Plans
Service cost	1,000,000	1,000,000	3,000,000	3,000,000
Interest cost	7,000,000	8,000,000	14,000,000	15,000,000
Expected return on plan assets	(7,000,000)	(6,000,000)	(13,000,000)	(12,000,000)
Amortization:
Prior service costs	1,000,000	1,000,000		1,000,000
Regulatory asset	0		1,000,000
Net (gain)/loss	2,000,000	1,000,000	4,000,000	3,000,000
Net periodic pension cost (income)	4,000,000	5,000,000	9,000,000	10,000,000
Postretirement Benefits \| GULF POWER CO
Pension Plans and Postretirement Plans
Service cost	1,000,000	1,000,000	1,000,000	1,000,000
Interest cost	0	0	1,000,000	1,000,000
Expected return on plan assets	(1,000,000)	(1,000,000)	(1,000,000)	(1,000,000)
Amortization:
Prior service costs	0	0		0
Regulatory asset	0		0
Net (gain)/loss	0	0	0	0
Net periodic pension cost (income)	0	0	1,000,000	1,000,000
Postretirement Benefits \| MISSISSIPPI POWER CO
Pension Plans and Postretirement Plans
Service cost	1,000,000	1,000,000	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	(1,000,000)	(1,000,000)	(1,000,000)	(1,000,000)
Amortization:
Prior service costs	0	0		0
Regulatory asset	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 \| 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	0	1,000,000	1,000,000
Interest cost	3,000,000	2,000,000	5,000,000	5,000,000
Expected return on plan assets	(2,000,000)	(1,000,000)	(4,000,000)	(3,000,000)
Amortization:
Prior service costs	0	0		(1,000,000)
Regulatory asset	2,000,000		3,000,000
Net (gain)/loss	0	1,000,000	0	2,000,000
Net periodic pension cost (income)	3,000,000	2,000,000	5,000,000	4,000,000
Postretirement Benefits \| Southern Company
Pension Plans and Postretirement Plans
Service cost	6,000,000	6,000,000	12,000,000	12,000,000
Interest cost	18,000,000	20,000,000	37,000,000	40,000,000
Expected return on plan assets	(17,000,000)	(17,000,000)	(34,000,000)	(33,000,000)
Amortization:
Prior service costs	1,000,000	1,000,000		3,000,000
Regulatory asset	0		3,000,000
Net (gain)/loss	4,000,000	5,000,000	7,000,000	7,000,000
Net periodic pension cost (income)	$ 12,000,000	$ 15,000,000	$ 25,000,000	$ 29,000,000

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

No definition available.

Name:	so_DefinedBenefitPlanNetPeriodicBenefitCostAmortizationAbstract
Namespace Prefix:	so_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The expense charged against earnings during the period to allocate the capitalized costs of regulatory assets over the periods expected to benefit from such costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.3)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_AmortizationOfRegulatoryAsset
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

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

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit plan.

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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.

+ References

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

+ Details

Name:	us-gaap_DefinedBenefitPlanExpectedFutureEmployerContributionsRemainderOfFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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.

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)(2)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanInterestCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

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

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanServiceCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=so_AlabamaPowerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=so_GeorgiaPowerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=so_GulfPowerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=so_MississippiPowerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=so_SouthernPowerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=so_SouthernCompanyGasMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConsolidatedEntitiesAxis=us-gaap_ParentCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: