Public Utilities, Maximum Base Rate

No definition available.

Name:	so_PublicUtilitiesMaximumBaseRate
Namespace Prefix:	so_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Public Utilities, Requested Rate Increase, Projected Test Year, Term

+ References

No definition available.

+ Details

Name:	so_PublicUtilitiesRequestedRateIncreaseProjectedTestYearTerm
Namespace Prefix:	so_
Data Type:	xbrli:durationItemType
Balance Type:	na
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_LossContingenciesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The net amount of all regulatory assets less all regulatory liabilities as of the end of the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

+ Details

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

- Definition

Percentage of public utility's requested equity to capital, including debt and equity.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesRequestedEquityCapitalStructurePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of public utility's requested rate increase (decrease) with regulatory agency.

+ References

No definition available.

+ Details

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

- Definition

Percentage of public utility's requested return on equity.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesRequestedReturnOnEquityPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Details

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

- Details

Name:	us-gaap_UnusualOrInfrequentItemAxis=so_COVID19Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

REGULATORY MATTERS - Southern Company Gas (Details) - USD ($) $ in Millions	Jul. 01, 2020	Jun. 01, 2020	Sep. 30, 2020
Nicor Gas
Loss Contingencies [Line Items]
Requested rate increase		$ 49.6
Requested rate increase, projected test year, period		12 months
Requested return on equity percentage		10.35%
Requested equity ratio percentage		54.00%
Atlanta Gas Light
Loss Contingencies [Line Items]
Requested rate increase	$ 37.6
Requested rate increase, projected test year, period	12 months
Maximum base rate	5.00%
COVID-19 \| Nicor Gas
Loss Contingencies [Line Items]
Regulatory assets			$ 13.0
COVID-19 \| Virginia Natural Gas
Loss Contingencies [Line Items]
Regulatory assets			$ 1.0