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Asset Impairment (Details)
$ in Millions, $ in Millions
 3 Months Ended 12 Months Ended
Sep. 30, 2021
CAD ($)
Dec. 31, 2021
CAD ($)
MW
Dec. 31, 2020
CAD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
CAD ($)
Oct. 01, 2019
MW
Disclosure of impairment loss recognised or reversed [line items]            
Asset impairment charge   $ 648 $ 84   $ 25  
Estimated salvage value   33        
Impairment charges   345 79      
Provision for decommissioning, restoration and rehabilitation costs $ 120          
Assets   9,226 9,747      
Gross carrying amount            
Disclosure of impairment loss recognised or reversed [line items]            
Write-downs (reversals of write-downs) of property, plant and equipment   $ 589 $ 81      
Discount rate, measurement input            
Disclosure of impairment loss recognised or reversed [line items]            
Significant unobservable input, assets   5,000,000.0 5,300,000      
Assets Held for Sale            
Disclosure of impairment loss recognised or reversed [line items]            
Estimated salvage value   $ 25        
Impairment charges   0 $ 0   15  
Energy Transition            
Disclosure of impairment loss recognised or reversed [line items]            
Asset impairment charge   191        
Keephills Unit 1 | Energy Transition            
Disclosure of impairment loss recognised or reversed [line items]            
Asset impairment charge   94        
Sundance Unit 4 | Energy Transition            
Disclosure of impairment loss recognised or reversed [line items]            
Asset impairment charge   56        
Centralia Thermal Facility            
Disclosure of impairment loss recognised or reversed [line items]            
Provision for decommissioning, restoration and rehabilitation costs         141  
Reversal of impairment loss         151  
Wind Facilities            
Disclosure of impairment loss recognised or reversed [line items]            
Impairment charges $ 10          
Wind Facilities | Level III | Investment property [member] | Non-recurring fair value measurement [member]            
Disclosure of impairment loss recognised or reversed [line items]            
Assets   65        
Kent Hills Wind L.P.            
Disclosure of impairment loss recognised or reversed [line items]            
Impairment charges   $ 2        
Capacity of facility (in megawatts) | MW   167        
Property, plant and equipment | Sundance Unit 3            
Disclosure of impairment loss recognised or reversed [line items]            
Impairment loss     70      
Land            
Disclosure of impairment loss recognised or reversed [line items]            
Impairment loss     9 $ 7    
Land | Gross carrying amount            
Disclosure of impairment loss recognised or reversed [line items]            
Write-downs (reversals of write-downs) of property, plant and equipment   $ 0 9      
Kaybob Cogeneration Project            
Disclosure of impairment loss recognised or reversed [line items]            
Capacity of facility (in megawatts) | MW           40
Mining assets | Highvale Mine            
Disclosure of impairment loss recognised or reversed [line items]            
Impairment charges   $ 195 $ 0   $ 0