Fair Value Measurements - Fair Value Measurements Calculated at Net Asset Value Per Share (Details)	9 Months Ended
Sep. 30, 2015 USD ($)
Foreign equity funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 109,000,000
Unfunded Commitments	$ 0
Redemption Notice Period	5 days
Equity - commingled funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 45,000,000
Unfunded Commitments	0
Debt - Commingled Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	16,000,000
Unfunded Commitments	$ 0
Redemption Notice Period	5 days
Other - commingled funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 5,000,000
Unfunded Commitments	0
Other - money market funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	16,000,000
Unfunded Commitments	0
Trust owned life insurance [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	112,000,000
Unfunded Commitments	$ 0
Redemption Notice Period	15 days
Money market funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 747,000,000
Unfunded Commitments	0
Alabama Power [Member] \| Equity - commingled funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	45,000,000
Unfunded Commitments	0
Alabama Power [Member] \| Debt - Commingled Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	16,000,000
Unfunded Commitments	$ 0
Redemption Notice Period	5 days
Alabama Power [Member] \| Trust owned life insurance [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 112,000,000
Unfunded Commitments	$ 0
Redemption Notice Period	15 days
Alabama Power [Member] \| Money market funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 484,000,000
Unfunded Commitments	0
Georgia Power [Member] \| Foreign equity funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	109,000,000
Unfunded Commitments	$ 0
Redemption Notice Period	5 days
Georgia Power [Member] \| Other - commingled funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 5,000,000
Unfunded Commitments	0
Georgia Power [Member] \| Other - money market funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	16,000,000
Unfunded Commitments	0
Georgia Power [Member] \| Money market funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	37,000,000
Unfunded Commitments	0
Gulf Power [Member] \| Money market funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	18,000,000
Unfunded Commitments	0
Mississippi Power [Member] \| Money market funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	64,000,000
Unfunded Commitments	0
Southern Power [Member] \| Money market funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	103,000,000
Unfunded Commitments	$ 0

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.

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)

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

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- Definition

Notice period the entity is required to deliver before it can redeem an investment calculated using net asset value per share, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Includes, but is not limited to, investments calculated by per unit, per membership interest, other equity or ownership interest and 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 (d)

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

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- 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

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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

+ Details

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