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Variable Interest Entities (Pipeline Investments) (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Variable Interest Entities [Line Items]    
Investments in joint ventures $ 1,328 $ 1,175
Variable Interest Entity, Not Primary Beneficiary [Member]    
Variable Interest Entities [Line Items]    
Variable interest entity, nonconsolidated, carrying amount, liabilities 28 29
Investments in joint ventures 966 919
Piedmont Natural Gas [Member]    
Variable Interest Entities [Line Items]    
Investments in joint ventures $ 63 61
Atlantic Coast Pipeline [Member]    
Variable Interest Entities [Line Items]    
Ownership percentage 47.00%  
Atlantic Coast Pipeline [Member] | Variable Interest Entity, Not Primary Beneficiary [Member]    
Variable Interest Entities [Line Items]    
Equity method investment, line of credit facility, maximum borrowing capacity   3,400
Investments in joint ventures $ 704 397
Atlantic Coast Pipeline [Member] | Variable Interest Entity, Not Primary Beneficiary [Member] | Revolving Credit Facility [Member]    
Variable Interest Entities [Line Items]    
Variable interest entity, entity maximum loss exposure, percent 47.00%  
Variable interest entity, nonconsolidated, carrying amount, liabilities $ 486  
Sabal Trail Transmission Pipeline [Member]    
Variable Interest Entities [Line Items]    
Investments in joint ventures $ 112  
Ownership percentage 7.50%  
Sabal Trail Transmission Pipeline [Member] | Variable Interest Entity, Not Primary Beneficiary [Member]    
Variable Interest Entities [Line Items]    
Investments in joint ventures   219
Constitution Pipeline [Member]    
Variable Interest Entities [Line Items]    
Ownership percentage 24.00%  
Constitution Pipeline [Member] | Variable Interest Entity, Not Primary Beneficiary [Member]    
Variable Interest Entities [Line Items]    
Equity method investment, OTTI $ 55  
Investments in joint ventures 25 81
Pipeline Investments [Member] | Variable Interest Entity, Not Primary Beneficiary [Member]    
Variable Interest Entities [Line Items]    
Investments in joint ventures $ 729 $ 697