SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Concentration of Credit Risk (Narrative) (Details) $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($) counterparty customer	Dec. 31, 2018 USD ($)
Concentration Risk [Line Items]
Receivables	$ 2,413	$ 2,630
Southern Company Gas
Concentration Risk [Line Items]
Receivables	$ 749	$ 952
Atlanta Gas Light
Concentration Risk [Line Items]
Number of customers in Georgia \| customer	16
Percent of highest month's estimated bill	200.00%
Credit Concentration Risk \| Wholesale gas services \| Accounts Receivable \| Southern Company Gas
Concentration Risk [Line Items]
Number of top counterparties \| counterparty	20
Credit risk exposure, measurement period	30 days
Concentration risk (as percent)	59.00%
Receivables	$ 218

Concentration Risk, Number of Customers.

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- Definition

Credit Risk Exposure, Measurement Period

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Represents the number of top counterparties.

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- Definition

Public Utilities, Annual Credit Security Support, Percentage Of Highest Bill

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- Definition

Amount, after allowance for credit loss, of right to consideration from customer for product sold and service rendered in normal course of business.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3))
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-Name Accounting Standards Codification
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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.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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-SubTopic 10
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