v3.10.0.1
Summary of Significant Accounting Policies - Concentration of Credit Risk (Narrative) (Details)
$ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
counterparty
customer
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