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Guarantees and Indemnifications (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Oct. 31, 2017
Guarantor Obligations [Line Items]      
Subsidiary debt guaranteed by parent $ 650 $ 650  
Unused letters of credit 60    
Face value of guarantees 23 $ 21  
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 63    
Standby Letters Of Credit [Member]      
Guarantor Obligations [Line Items]      
Maximum potential amount of future payments associated with guarantees 454    
Performance Guarantee [Member]      
Guarantor Obligations [Line Items]      
Maximum potential of future amounts associated with guarantees with expiration dates 248    
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 296    
Financial Guarantee [Member] | Atlantic Coast Pipeline [Member]      
Guarantor Obligations [Line Items]      
Maximum potential amount of future payments associated with guarantees $ 677    
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