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Assets Held for Sale - Additional Information (Detail)
CAD in Millions
 12 Months Ended
Dec. 31, 2017
CAD
Disclosure of assets held for sale [line items]  
Break fee paid for sale of asset CAD 28
Waneta Dam [member]  
Disclosure of assets held for sale [line items]  
Percentage of sales interest in joint operation 66.67%
Proceeds from joint operation CAD 1,200
Lease period 20-year
Additional lease period 10 years
Break fee paid for sale of asset CAD (28)