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Investment in Cenovus Energy (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
3 Months Ended 5 Months Ended 9 Months Ended
Sep. 30, 2018
May 17, 2017
Sep. 30, 2018
Jan. 01, 2018
Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]          
Investment in Cenovus Energy $ 2,086   $ 2,086   $ 1,899
Accumulated Other Comprehensive Income , Net of Tax (5,442)   (5,442)   (5,518)
Adjustments for New Accounting Pronouncement [Member]          
Schedule Of Available For Sale Securities [Line Items]          
Accumulated Other Comprehensive Income , Net of Tax       $ (5,460) (5,518)
Common Stock [Member]          
Schedule Of Available For Sale Securities [Line Items]          
Ivnestment in Equity Security Cost   $ 1,960      
Common Stock [Member] | Foster Creek Christina Lake (FCCL) and western Canada gas properties [Member]          
Schedule Of Available For Sale Securities [Line Items]          
Shares Exchange for Assets Held for Sale   208      
Investment Owned, Common Stock, percent of Issued and Outstanding   16.90%      
Investment In Equity Security Fair Value 2,090 $ 1,960 $ 2,090   $ 1,900
Investment Owned, Common Stock, price per share   $ 9.41 $ 10.03    
Unrealized Gain (Loss) on Securities $ (73)   $ 187    
Common Stock [Member] | ASU 2016-01 Accounting for Cenovus Shares [Member] | Foster Creek Christina Lake (FCCL) and western Canada gas properties [Member]          
Schedule Of Available For Sale Securities [Line Items]          
Accumulated Other Comprehensive Income , Net of Tax       $ (58)