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Summary of Significant Accounting Policies - Additional Information (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2018
CAD ($)
Nueva Union [member]  
Disclosure of significant accounting policies [line items]  
Ownership interest in joint venture 50.00%
Galore Creek Partnership [member]  
Disclosure of significant accounting policies [line items]  
Ownership interest in joint operation 50.00%
Fort Hills Energy Limited Partnership [member]  
Disclosure of significant accounting policies [line items]  
Ownership interest in joint operation 21.30%
Consideration for increased portion of ownership interest $ 58
Antamina [member]  
Disclosure of significant accounting policies [line items]  
Ownership interest in joint operation 22.50%
Bottom of range [member] | Fort Hills Energy Limited Partnership [member]  
Disclosure of significant accounting policies [line items]  
Ownership interest in joint operation 20.89%
Top of range [member] | Fort Hills Energy Limited Partnership [member]  
Disclosure of significant accounting policies [line items]  
Ownership interest in joint operation 21.30%