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Commitments - Additional Information (Detail)
CAD in Millions, $ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2017
CAD
Dec. 31, 2017
USD ($)
Dec. 31, 2016
CAD
Dec. 31, 2016
USD ($)
Disclosure of commitments [Line Items]          
Amount expected to be incurred   CAD 766      
Expected period   1 year 1 year    
Amount on lease rentals   CAD 113   CAD 93  
Royalty expense   453   312  
Within one year [member]          
Disclosure of commitments [Line Items]          
Amount expected to be incurred   520      
Within two to five years [member]          
Disclosure of commitments [Line Items]          
Amount expected to be incurred   246      
Office premises [member]          
Disclosure of commitments [Line Items]          
Amount on lease rentals   13   10  
Mining equipment [member]          
Disclosure of commitments [Line Items]          
Amount on lease rentals   60   36  
Rail facilities [member]          
Disclosure of commitments [Line Items]          
Amount on lease rentals   12   10  
Road and port facilities [member]          
Disclosure of commitments [Line Items]          
Amount on lease rentals   28   37  
Quebrada Blanca Phase 2 [member]          
Disclosure of commitments [Line Items]          
Amount expected to be incurred   323      
Fort Hills [member]          
Disclosure of commitments [Line Items]          
Amount expected to be incurred   CAD 196      
Ownership interest in joint operation   20.89% 20.89%    
Antamina [member]          
Disclosure of commitments [Line Items]          
Royalty percentage   7.40% 7.40%    
Royalty expense   CAD 28   CAD 17  
Steelmaking Coal Operations [member]          
Disclosure of commitments [Line Items]          
Amount expected to be incurred   CAD 175      
Red Dog Royalty [member]          
Disclosure of commitments [Line Items]          
Royalty percentage   35.00% 35.00%    
Maximum net proceeds of production royalty   50.00% 50.00%    
Increase in net proceeds of production royalty 35.00% 5.00% 5.00%    
Royalty expense | $     $ 324   $ 213
Red Dog Road and Port lease [member] | Over next fifteen years [member]          
Disclosure of commitments [Line Items]          
Minimum annual user fee | $     18    
Red Dog Road and Port lease [member] | Over next seven years [member]          
Disclosure of commitments [Line Items]          
Minimum annual user fee | $     6    
Red Dog Road and Port lease [member] | Over twenty two years [member]          
Disclosure of commitments [Line Items]          
Total remaining user fee | $     $ 312    
Other Operations [member]          
Disclosure of commitments [Line Items]          
Amount expected to be incurred   CAD 72      
Bottom of range [member]          
Disclosure of commitments [Line Items]          
Capital expenditure commitments expected period   2 years 2 years    
Top of range [member]          
Disclosure of commitments [Line Items]          
Capital expenditure commitments expected period   5 years 5 years    
Lease term       20 years 20 years