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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
Jan. 26, 2017
Sep. 30, 2018
Jun. 30, 2017
Sep. 30, 2018
Dec. 31, 2018
Dec. 31, 2016
May 31, 2018
Disclosure of impairment loss recognised or reversed [line items]                
Write-downs (reversals of write-downs) of property, plant and equipment           $ 23    
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              
Discount rate               7.00%
Sundance Unit 1                
Disclosure of impairment loss recognised or reversed [line items]                
Impairment loss     $ 38 $ 20        
Wintering Hills                
Disclosure of impairment loss recognised or reversed [line items]                
Equity participation percentage   51.00%            
Cash transferred   $ 61            
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 | 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