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Income Taxes - Additional Information (Detail)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2018
CAD ($)
Dec. 31, 2017
CAD ($)
Dec. 31, 2018
USD ($)
Disclosure of Income Taxes [line items]      
Unrecognized deferred tax liabilities $ 745 $ 694  
Unused tax losses $ 2,910 3,630  
Loss carryforwards expiration, start year 2029    
Loss carryforwards expiration, end year 2038    
Cumulative Canadian development expenses $ 685 981  
Development expenses deductible percentage 30.00%    
Federal investment tax credits $ 106 104  
Investment tax credits, start year 2022    
Investment tax credits, end year 2038    
Antamina [member]      
Disclosure of Income Taxes [line items]      
Ownership interest in joint operation 22.50%    
2013 [member] | Antamina [member]      
Disclosure of Income Taxes [line items]      
Current tax debt, including interest and penalties $ 40   $ 30
Charge to earnings consisting of interest and penalties 20   15
2013, 2014, and 2015 [member] | Antamina [member]      
Disclosure of Income Taxes [line items]      
Current tax debt, including interest and penalties 125   94
Charge to earnings consisting of interest and penalties 60   $ 45
Unused Tax Credits [member]      
Disclosure of Income Taxes [line items]      
Unrecognized deferred tax assets $ 239 $ 231