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Goodwill (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2018
CAD ($)
MWh
Dec. 31, 2017
CAD ($)
MWh
Disclosure of detailed information about intangible assets [line items]    
Goodwill (Note 18) $ 464 $ 463
Hydro    
Disclosure of detailed information about intangible assets [line items]    
Goodwill (Note 18) 259 259
Wind and Solar    
Disclosure of detailed information about intangible assets [line items]    
Goodwill (Note 18) 175 174
Energy Marketing    
Disclosure of detailed information about intangible assets [line items]    
Goodwill (Note 18) $ 30 $ 30
Minimum    
Disclosure of detailed information about intangible assets [line items]    
Electricity prices | MWh 5.83 22
Discount rate used to calculate goodwill impairment 5.30% 5.50%
Maximum    
Disclosure of detailed information about intangible assets [line items]    
Electricity prices | MWh 179 218
Discount rate used to calculate goodwill impairment 6.20% 6.00%