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Asset Impairment Charges and Reversals (Details)
$ in Millions, $ in Millions
 3 Months Ended 12 Months Ended
Sep. 30, 2020
CAD ($)
Sep. 30, 2018
CAD ($)
Dec. 31, 2020
CAD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
CAD ($)
Dec. 31, 2018
CAD ($)
Dec. 31, 2012
CAD ($)
Jun. 28, 2018
Disclosure of impairment loss recognised or reversed [line items]                
Impairment loss $ 70              
Write-downs (reversals of write-downs) of property, plant and equipment     $ 0   $ 18 $ 23    
Gross carrying amount                
Disclosure of impairment loss recognised or reversed [line items]                
Write-downs (reversals of write-downs) of property, plant and equipment     81   (431)      
Disposal groups classified as held for sale                
Disclosure of impairment loss recognised or reversed [line items]                
Impairment loss         15      
Sundance Unit 2                
Disclosure of impairment loss recognised or reversed [line items]                
Impairment loss   $ 38            
Lakeswind                
Disclosure of impairment loss recognised or reversed [line items]                
Impairment loss           12    
Lakeswind | Discount rates                
Disclosure of impairment loss recognised or reversed [line items]                
Significant unobservable input, assets               0.07
Property, plant and equipment | Centralia Thermal Facility                
Disclosure of impairment loss recognised or reversed [line items]                
Impairment loss             $ 347  
Provision for decommissioning, restoration and rehabilitation costs         141      
Reversal of impairment loss         151      
Property, plant and equipment | Lakeswind                
Disclosure of impairment loss recognised or reversed [line items]                
Impairment loss           11    
Property, plant and equipment | Sundance Unit 3                
Disclosure of impairment loss recognised or reversed [line items]                
Impairment loss 70              
Property, plant and equipment | BC Hydro Facility                
Disclosure of impairment loss recognised or reversed [line items]                
Impairment loss $ 2              
Property, plant and equipment | Centralia mine and Sundance Unit 1                
Disclosure of impairment loss recognised or reversed [line items]                
Provision for decommissioning, restoration and rehabilitation costs     3          
Land | Gross carrying amount                
Disclosure of impairment loss recognised or reversed [line items]                
Write-downs (reversals of write-downs) of property, plant and equipment     9   $ 0      
Land | Gross carrying amount | Centralia Segment                
Disclosure of impairment loss recognised or reversed [line items]                
Impairment charges     $ (9) $ (7)        
Intangible assets | Lakeswind                
Disclosure of impairment loss recognised or reversed [line items]                
Impairment loss           $ 1