Summary of Significant Accounting Policies - Concentration of Credit Risk (Narrative) (Details) $ in Millions |
12 Months Ended |
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Dec. 31, 2018
USD ($)
counterparty
customer
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| Concentration Risk [Line Items] | |
| Receivables | $ 2,630 |
| Southern Company Gas | |
| Concentration Risk [Line Items] | |
| Receivables | $ 952 |
| Atlanta Gas Light | |
| Concentration Risk [Line Items] | |
| Number of customers in Georgia | customer | 15 |
| 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 |
| Concentration risk (as percent) | 48.00% |
| Receivables | $ 298 |
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- Definition Concentration Risk, Number of Customers. No definition available.
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- Definition Represents the number of top counterparties. No definition available.
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- Definition Public Utilities, Annual Credit Security Support, Percentage Of Highest Bill No definition available.
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- Definition For an unclassified balance sheet, the amount due from customers or clients for goods or services that have been delivered or sold in the normal course of business, reduced to their estimated net realizable fair value by an allowance established by the entity of the amount it deems uncertain of collection. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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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. No definition available.
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- Definition 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. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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