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Fair Value Measurement - FV Hierarchy (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investment in Cenovus Energy $ 2,086 $ 1,899
Commodity derivative asset, gross $ 301 311
Fair Value, Assets, Level 2 to Level 1 Transfers, Description At the end of the fourth quarter of 2017, our investment in Cenovus Energy transferred from Level 2 to Level 1 due to the lapsing of trading restrictions.  
Fair Value, Measurements, Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investment in Cenovus Energy $ 2,086 1,899
Commodity derivative asset, gross 301 311
Total assets 2,387 2,210
Commodity derivative liability, gross 315 310
Total liabilities 315 310
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investment in Cenovus Energy 2,086 1,899
Commodity derivative asset, gross 186 175
Total assets 2,272 2,074
Commodity derivative liability, gross 176 158
Total liabilities 176 158
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investment in Cenovus Energy 0 0
Commodity derivative asset, gross 97 106
Total assets 97 106
Commodity derivative liability, gross 103 111
Total liabilities 103 111
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investment in Cenovus Energy 0 0
Commodity derivative asset, gross 18 30
Total assets 18 30
Commodity derivative liability, gross 36 41
Total liabilities $ 36 $ 41