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Exploration and Evaluation Assets - Detailed information about exploration and evaluation assets (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure Of Detailed Information About Exploration For And Evaluation Of Mineral Resources [Line Items]      
Beginning balance $ 2,637 $ 2,632  
Additions 71 502  
Acquisition of Devon assets (note 7) 91    
Transfers to property, plant and equipment (219) (397)  
Disposals/derecognitions and other   (100)  
Foreign exchange adjustments (1)    
Ending balance 2,579 2,637 $ 2,632
North America      
Disclosure Of Detailed Information About Exploration For And Evaluation Of Mineral Resources [Line Items]      
Beginning balance 2,348 2,282  
Additions 38 245  
Acquisition of Devon assets (note 7) 91    
Transfers to property, plant and equipment (219) (175)  
Disposals/derecognitions and other   (4) (1)
Foreign exchange adjustments 0    
Ending balance 2,258 2,348 2,282
North Sea      
Disclosure Of Detailed Information About Exploration For And Evaluation Of Mineral Resources [Line Items]      
Beginning balance 0 0  
Additions 0 0  
Acquisition of Devon assets (note 7) 0    
Transfers to property, plant and equipment 0 0  
Disposals/derecognitions and other   0  
Foreign exchange adjustments 0    
Ending balance 0 0 0
Offshore Africa      
Disclosure Of Detailed Information About Exploration For And Evaluation Of Mineral Resources [Line Items]      
Beginning balance 37 91  
Additions 33 35  
Acquisition of Devon assets (note 7) 0    
Transfers to property, plant and equipment 0 0  
Disposals/derecognitions and other   (89)  
Foreign exchange adjustments (1)    
Ending balance 69 37 91
Oil Sands Mining and Upgrading      
Disclosure Of Detailed Information About Exploration For And Evaluation Of Mineral Resources [Line Items]      
Beginning balance 252 259  
Additions 0 222  
Acquisition of Devon assets (note 7) 0    
Transfers to property, plant and equipment 0 (222)  
Disposals/derecognitions and other   (7)  
Foreign exchange adjustments 0    
Ending balance $ 252 $ 252 $ 259