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Variable Interest Entities (Pipeline Investments) (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Variable Interest Entities [Line Items]    
Investments in Joint Ventures $ 1,168 $ 1,175
Variable Interest Entity, Not Primary Beneficiary [Member]    
Variable Interest Entities [Line Items]    
Variable Interest Entity, Nonconsolidated, Carrying Amount, Liabilities 26 29
Investments in Joint Ventures 803 919
Piedmont Natural Gas [Member]    
Variable Interest Entities [Line Items]    
Investments in Joint Ventures $ 62 61
Atlantic Coast Pipeline [Member]    
Variable Interest Entities [Line Items]    
Equity Method Investment, 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 $ 541 397
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 $ 412  
Sabal Trail Transmission Pipeline [Member]    
Variable Interest Entities [Line Items]    
Investments in Joint Ventures $ 110  
Equity Method Investment, 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]    
Equity Method Investment, Ownership Percentage 24.00%  
Constitution Pipeline [Member] | Variable Interest Entity, Not Primary Beneficiary [Member]    
Variable Interest Entities [Line Items]    
Equity Method Investment, Other than Temporary Impairment $ 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 $ 566 $ 697