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Update to Significant Accounting Policies, Critical Accounting Judgements and Key Sources of Estimation Uncertainty (Details)
$ / shares in Units, $ in Billions
 9 Months Ended
Sep. 30, 2021
CAD ($)
$ / shares
WRB Refining LP  
Disclosure of changes in accounting estimates [line items]  
Proportion of ownership interest in joint operation 50.00%
Sunrise Oil Sands Partnership  
Disclosure of changes in accounting estimates [line items]  
Proportion of ownership interest in joint operation 50.00%
BP-Husky Refining LLC  
Disclosure of changes in accounting estimates [line items]  
Proportion of ownership interest in joint operation 50.00%
Information technology assets  
Disclosure of changes in accounting estimates [line items]  
Useful life measured as period of time, property, plant and equipment 3 years
Land improvements and buildings | Bottom of range  
Disclosure of changes in accounting estimates [line items]  
Useful life measured as period of time, property, plant and equipment 15 years
Land improvements and buildings | Top of range  
Disclosure of changes in accounting estimates [line items]  
Useful life measured as period of time, property, plant and equipment 40 years
Office improvements and buildings | Bottom of range  
Disclosure of changes in accounting estimates [line items]  
Useful life measured as period of time, property, plant and equipment 3 years
Office improvements and buildings | Top of range  
Disclosure of changes in accounting estimates [line items]  
Useful life measured as period of time, property, plant and equipment 15 years
Refining equipment | Bottom of range  
Disclosure of changes in accounting estimates [line items]  
Useful life measured as period of time, property, plant and equipment 10 years
Refining equipment | Top of range  
Disclosure of changes in accounting estimates [line items]  
Useful life measured as period of time, property, plant and equipment 60 years
Other | Bottom of range  
Disclosure of changes in accounting estimates [line items]  
Useful life measured as period of time, property, plant and equipment 3 years
Other | Top of range  
Disclosure of changes in accounting estimates [line items]  
Useful life measured as period of time, property, plant and equipment 60 years
The Plan  
Disclosure of changes in accounting estimates [line items]  
Target share price (in CAD per share) | $ / shares $ 20
Target trading days 20 days
The Incentive Plan  
Disclosure of changes in accounting estimates [line items]  
Target run-rate synergies | $ $ 1.0
The Incentive Plan | Minimum  
Disclosure of changes in accounting estimates [line items]  
Cash award percentage of base salary 15.00%
The Incentive Plan | Maximum  
Disclosure of changes in accounting estimates [line items]  
Cash award percentage of base salary 30.00%