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Provisions and Other Liabilities - Additional Information (Detail) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of other provisions [line items]    
Additional study and environmental costs arising from legal obligations through other provisions $ 33 $ 121
Provisions for decommissioning and restoration 1,160  
Provision for water management costs 454  
Provisions for decommissioning and restoration expenditures of closed operations $ 249 $ 270
Decommissioning and Restoration Provisions [member]    
Disclosure of other provisions [line items]    
Inflation rate 2.00%  
Bottom of range [member] | Decommissioning and Restoration Provisions [member]    
Disclosure of other provisions [line items]    
Nominal discount rate 6.49%  
Top of range [member] | Decommissioning and Restoration Provisions [member]    
Disclosure of other provisions [line items]    
Nominal discount rate 7.99%