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Asset Impairment Charges and Reversals (Details) - CAD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Jun. 28, 2018
Dec. 31, 2019
Jun. 30, 2017
Sep. 30, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2016
Dec. 31, 2012
Disclosure of impairment loss recognised or reversed [line items]                
Write-downs (reversals of write-downs) of property, plant and equipment         $ 18 $ 23    
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              
Sundance Unit 1                
Disclosure of impairment loss recognised or reversed [line items]                
Impairment loss     $ 20          
Wintering Hills | Wind and Solar                
Disclosure of impairment loss recognised or reversed [line items]                
Decrease through classified as held for sale, investment property             $ 28  
Property, plant and equipment | Centralia Plant CGU                
Disclosure of impairment loss recognised or reversed [line items]                
Impairment loss               $ 347
Reversal of impairment loss         151      
Provision for decommissioning, restoration and rehabilitation costs   $ 141     $ 141      
Property, plant and equipment | Lakeswind                
Disclosure of impairment loss recognised or reversed [line items]                
Impairment loss 11              
Intangible assets | Lakeswind                
Disclosure of impairment loss recognised or reversed [line items]                
Impairment loss $ 1