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Accounting for Financial Instruments - Additional Information (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2017
CAD
Dec. 31, 2016
CAD
Dec. 31, 2016
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Disclosure of nature and extent of risks arising from financial instruments [line items]          
Pre-tax gain loss from financial instruments due to interest rate change CAD 10,000,000 CAD 13,000,000      
Gain on non-operating income (expense) 51,000,000 113,000,000      
Sale and repurchase agreements [member]          
Disclosure of nature and extent of risks arising from financial instruments [line items]          
Outstanding settlements receivable 687,000,000 631,000,000      
Outstanding settlements payable 39,000,000 43,000,000      
Embedded derivatives [member]          
Disclosure of nature and extent of risks arising from financial instruments [line items]          
Fair value of embedded derivative CAD 9,000,000 2,000,000      
Percentage of ongoing payment containing embedded derivative 15        
Percentage of ongoing payment containing embedded derivative 5        
Fair value of embedded derivative CAD 3,000,000 1,000,000      
Notes Due 2024 [member] | Embedded derivatives [member]          
Disclosure of nature and extent of risks arising from financial instruments [line items]          
Fair value of embedded derivative 108,000,000 78,000,000      
Contingent Zinc Escalation Payment Embedded Derivative [member] | Embedded derivatives [member]          
Disclosure of nature and extent of risks arising from financial instruments [line items]          
Fair value of embedded derivative | $       $ 43 $ 20
Derivatives not designated as hedges [member]          
Disclosure of nature and extent of risks arising from financial instruments [line items]          
Fair value of commodity swaps | $       6  
Currency risk [member]          
Disclosure of nature and extent of risks arising from financial instruments [line items]          
Strengthening/Weakening of Canadian dollar against the U.S. dollar 0.10        
Pre-tax gain loss from financial instruments due to foreign currency translation (3,000,000) (10,000,000)      
Pre-tax gain loss in other comprehensive income from translation of foreign operations (157,000,000) CAD (11,000,000)      
Commodity Price Risk [member]          
Disclosure of nature and extent of risks arising from financial instruments [line items]          
Percentage of change in commodity price   10 10    
Change in profit loss due to change in commodity price CAD 26,000,000   $ 45    
Currency risk [member]          
Disclosure of nature and extent of risks arising from financial instruments [line items]          
Currency denominated debt | $       $ 4,100