FAIR VALUE MEASUREMENTS - Narrative (Details) $ in Millions	Dec. 31, 2019 USD ($) $ / MMBTU	Dec. 31, 2018 USD ($)
Private equity \| Alabama Power
Fair Value, Option, Quantitative Disclosures [Line Items]
Alternative investment	$ 56	$ 45
Unfunded commitments	70	$ 50
Energy-related derivatives \| Significant Unobservable Inputs (Level 3) \| Southern Company Gas
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, at fair value, net	$ 14
Minimum \| Measurement Input, Commodity Forward Price \| Valuation, Market Approach \| Energy-related derivatives \| Significant Unobservable Inputs (Level 3) \| Southern Company Gas
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative asset net, measurement input \| $ / MMBTU	1.54
Maximum \| Measurement Input, Commodity Forward Price \| Valuation, Market Approach \| Energy-related derivatives \| Significant Unobservable Inputs (Level 3) \| Southern Company Gas
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative asset net, measurement input \| $ / MMBTU	2.92

Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 35
-Paragraph 54B
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Value of input used to measure net derivative asset (liability).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
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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://fasb.org/us-gaap/role/ref/legacyRef
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-Topic 820
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-Section 50
-Paragraph 6A
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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.

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