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Asset and Goodwill Impairment Testing - Additional Information (Detail)
 12 Months Ended
Dec. 31, 2017
CAD
Dec. 31, 2016
CAD
Oct. 31, 2017
CAD
WCS oil [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Percentage of discount rate applied to cash flow projections 5.20% 5.50%  
Percentage of nominal post-tax discount rate 7.30% 7.60%  
Steelmaking Coal [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Recoverable amount of CGU exceeding carrying value     CAD 6,200,000,000
Decrease in price percentage     14.00%
Mining operations and goodwill [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Percentage of discount rate applied to cash flow projections 5.90% 5.80%  
Percentage of nominal post-tax discount rate 8.00% 7.90%  
Fort Hills Project [member] | WCS oil [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Decrease in assumption 1    
Amount by which recoverable amount is reduced when key assumption changes for commodity price CAD 140,000,000    
Adjustment to interest rate basis 0.25%    
Fort Hills Project [member] | WCS oil [member] | Discount rate [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Amount by which recoverable amount is reduced when key assumption changes for discount rate CAD 150,000,000    
Fort Hills Project [member] | WCS oil [member] | Canada, Dollars [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Decrease in assumption 0.01    
Amount by which recoverable amount is reduced when key assumption changes for foreign exchange rate CAD 50,000,000    
Steelmaking Coal Operations [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Cash flow period of mines Cash flows cover periods of 15 to 42 years    
Quebrada Blanca [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Cash flow period of mines Cash flows cover a period of 29 years    
Recoverable amount of CGU exceeding carrying value CAD 1,200,000,000    
Decrease in price percentage 6.00%    
Period over which recoverable amount will be equal to carrying value 3.5-year    
Steelmaking coal CGU [member] | Pretax [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Impairment reversal CAD 207,000,000    
Steelmaking coal CGU [member] | After tax [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Impairment reversal 131,000,000    
Fort Hills oil sands project [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Recoverable amount net of tax CAD 3,700,000,000 CAD 2,500,000,000  
Cash flow period of mines 49 years 44 years  
Fort Hills oil sands project [member] | Pretax [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Impairment charge CAD 0 CAD 222,000,000  
Fort Hills oil sands project [member] | After tax [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Impairment charge   164,000,000  
Other [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Impairment charge   72,000,000  
Other [member] | Quebrada Blanca assets [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Impairment charge CAD 44,000,000    
Other [member] | Trail operations [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Impairment charge   46,000,000  
Other [member] | Wintering hills wind power facility [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Impairment charge   CAD 26,000,000