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Guarantees and Indemnifications(Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Oct. 31, 2017
Dec. 31, 2016
Guarantor Obligations [Line Items]      
Unused letters of credit $ 66    
Face value of guarantees 21   $ 13
Atlantic Coast Pipeline [Member]      
Guarantor Obligations [Line Items]      
Equity Method Investment Line Of Credit Facility Maximum Borrowing Capacity   $ 3,400  
Surety Bond [Member]      
Guarantor Obligations [Line Items]      
Maximum potential amount of future payments associated with guarantees 81    
Standby Letters Of Credit [Member]      
Guarantor Obligations [Line Items]      
Maximum potential amount of future payments associated with guarantees 449    
Performance Guarantee [Member]      
Guarantor Obligations [Line Items]      
Maximum potential of future amounts associated with guarantees with expiration dates 281    
Performance Guarantee [Member] | Less Than Wholly Owned Entity [Member]      
Guarantor Obligations [Line Items]      
Maximum potential amount of future payments associated with guarantees 11    
Performance Guarantee [Member] | Third Parties, Unconsolidated Affiliates and Less Than Wholly Owned Consolidated Entities [Member]      
Guarantor Obligations [Line Items]      
Maximum potential amount of future payments associated with guarantees 326    
Financial Guarantee [Member] | Atlantic Coast Pipeline [Member]      
Guarantor Obligations [Line Items]      
Maximum potential amount of future payments associated with guarantees $ 312    
Guarantee Obligations Maximum Exposure Percentage 47.00%    
Spectra Capital [Member] | Performance Guarantee [Member]      
Guarantor Obligations [Line Items]      
Maximum potential amount of future payments associated with guarantees $ 205    
Progress Energy [Member] | Indemnification Agreement [Member]      
Guarantor Obligations [Line Items]      
Maximum potential amount of future payments associated with guarantees $ 89