Fair Value Measurements - Fair Value Measurements Calculated at Net Asset Value Per Share (Details) - Jun. 30, 2015 - USD ($)	Total
Foreign equity funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	$ 125,000,000
Unfunded Commitments	$ 0
Redemption Frequency	Monthly
Redemption Notice Period	5 days
Redemption Notice Period, Description
Equity - commingled funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	$ 50,000,000
Unfunded Commitments	$ 0
Redemption Frequency	Daily
Redemption Notice Period, Description
Debt - Commingled Funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	$ 16,000,000
Unfunded Commitments	$ 0
Redemption Frequency	Daily
Redemption Notice Period	5 days
Redemption Notice Period, Description
Other - commingled funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	$ 3,000,000
Unfunded Commitments	$ 0
Redemption Frequency	Daily
Redemption Notice Period, Description
Other - money market funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	$ 53,000,000
Unfunded Commitments	$ 0
Redemption Frequency	Daily
Redemption Notice Period, Description
Trust owned life insurance [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	$ 117,000,000
Unfunded Commitments	$ 0
Redemption Frequency	Daily
Redemption Notice Period	15 days
Redemption Notice Period, Description
Money market funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	$ 533,000,000
Unfunded Commitments	$ 0
Redemption Frequency	Daily
Redemption Notice Period, Description
Alabama Power [Member] \| Equity - commingled funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	$ 50,000,000
Unfunded Commitments	$ 0
Redemption Frequency	Daily
Redemption Notice Period, Description
Alabama Power [Member] \| Debt - Commingled Funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	$ 16,000,000
Unfunded Commitments	$ 0
Redemption Frequency	Daily
Redemption Notice Period	5 days
Redemption Notice Period, Description
Alabama Power [Member] \| Trust owned life insurance [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	$ 117,000,000
Unfunded Commitments	$ 0
Redemption Frequency	Daily
Redemption Notice Period	15 days
Redemption Notice Period, Description
Alabama Power [Member] \| Money market funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	$ 81,000,000
Unfunded Commitments	$ 0
Redemption Frequency	Daily
Redemption Notice Period, Description
Georgia Power [Member] \| Foreign equity funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	$ 125,000,000
Unfunded Commitments	$ 0
Redemption Frequency	Monthly
Redemption Notice Period	5 days
Redemption Notice Period, Description
Georgia Power [Member] \| Other - commingled funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	$ 3,000,000
Unfunded Commitments	$ 0
Redemption Frequency	Daily
Redemption Notice Period, Description
Georgia Power [Member] \| Other - money market funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	$ 53,000,000
Unfunded Commitments	$ 0
Redemption Frequency	Daily
Redemption Notice Period, Description
Gulf Power [Member] \| Money market funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	$ 18,000,000
Unfunded Commitments	$ 0
Redemption Frequency	Daily
Redemption Notice Period, Description
Mississippi Power [Member] \| Money market funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	$ 182,000,000
Unfunded Commitments	$ 0
Redemption Frequency	Daily
Redemption Notice Period, Description
Southern Power [Member] \| Money market funds [Member]
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments
Fair Value	$ 206,000,000
Unfunded Commitments	$ 0
Redemption Frequency	Daily
Redemption Notice Period, Description

Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Redemption Notice Periods

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- Definition

Fair Value Investments Entities that Calculate Redemption Notice Period

+ References

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Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments.

+ References

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- Definition

Fair value of investments in certain entities that calculate net asset value per share, including, but not limited to, by unit, membership interest, or other ownership interest. Investments include, but are not limited to, investments in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 6A

-Subparagraph (a)

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- Definition

States the frequency (for example, annually, semi-annually, quarterly) with which the entity can redeem its investment, or portion thereof, for each of those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments), by major category.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 6A

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=SL6742756-110258

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- Definition

The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 6A

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=SL6742756-110258

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