Summary of Significant Accounting Policies - Revenues (Narrative) (Details)	12 Months Ended
Dec. 31, 2018
Southern Company Gas
Disaggregation of Revenue [Line Items]
Period for collection of revenue prior to billings	24 months

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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