Supplementary Oil And Gas Information (Costs Not Subject to Depletion Or Amortization) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Capitalized Costs of Unproved Properties Excluded from Amortization, Cumulative	$ 4,480	$ 5,198
Canada [Member]
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Capitalized Costs of Unproved Properties Excluded from Amortization, Cumulative	311	285
United States [Member]
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Capitalized Costs of Unproved Properties Excluded from Amortization, Cumulative	$ 4,169	$ 4,913