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Variable Interest Entities (Pipeline Investments) (Details) - USD ($)
$ in Millions
3 Months Ended
Apr. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Variable Interest Entities [Line Items]      
Investments in Joint Ventures   $ 1,189 $ 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   $ 951 919
Variable Interest Entity, Not Primary Beneficiary [Member] | Atlantic Coast Pipeline [Member]      
Variable Interest Entities [Line Items]      
Variable Interest Entity Ownership Interest   47.00%  
Equity Method Investment Line Of Credit Facility Maximum Borrowing Capacity     3,400
Investments in Joint Ventures   $ 474 397
Variable Interest Entity, Not Primary Beneficiary [Member] | Atlantic Coast Pipeline [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   $ 346  
Variable Interest Entity, Not Primary Beneficiary [Member] | Sabal Trail Transmission Pipeline [Member]      
Variable Interest Entities [Line Items]      
Variable Interest Entity Ownership Interest   7.50%  
Investments in Joint Ventures   $ 223 219
Variable Interest Entity, Not Primary Beneficiary [Member] | Constitution Pipeline [Member]      
Variable Interest Entities [Line Items]      
Equity Method Investment, Other than Temporary Impairment   $ 55  
Variable Interest Entity Ownership Interest   24.00%  
Investments in Joint Ventures   $ 26 81
Variable Interest Entity, Not Primary Beneficiary [Member] | Pipeline Investments [Member]      
Variable Interest Entities [Line Items]      
Variable Interest Entity, Nonconsolidated, Carrying Amount, Liabilities   11 13
Investments in Joint Ventures   723 697
Piedmont Natural Gas [Member]      
Variable Interest Entities [Line Items]      
Investments in Joint Ventures   $ 62 $ 61
Subsequent Event [Member] | Variable Interest Entity, Not Primary Beneficiary [Member] | Sabal Trail Transmission Pipeline [Member]      
Variable Interest Entities [Line Items]      
Equity Method Investment Long Term Debt $ 1,500    
Proceeds from Equity Method Investment, Distribution, Return of Capital $ 112