Fair Value Measurements - Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Long-term debt:
Long-term Debt	$ 44,446	$ 44,462
Southern Company
Long-term debt:
Carrying Amount	47,479
Fair Value	48,836
ALABAMA POWER CO
Long-term debt:
Carrying Amount	7,626
Fair Value	8,093
Long-term Debt	7,429	7,628
GEORGIA POWER CO
Long-term debt:
Carrying Amount	11,402
Fair Value	11,851
Long-term Debt	10,797	11,073
GULF POWER CO
Long-term debt:
Carrying Amount	1,285
Fair Value	1,317
Long-term Debt	1,285	1,285
MISSISSIPPI POWER CO
Long-term debt:
Carrying Amount	1,781
Fair Value	1,790
Long-term Debt	1,567	1,097
SOUTHERN POWER CO
Long-term debt:
Carrying Amount	5,878
Fair Value	6,006
Long-term Debt	5,108	5,071
SOUTHERN Co GAS
Long-term debt:
Carrying Amount	6,036
Fair Value	6,276
Long-term Debt	$ 5,859	$ 5,891

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13476-108611

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- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Amount of long-term debt and capital lease obligation due after one year or beyond the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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