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Price Risk Management Net volumes related to price risk management activities (Details)
MWh in Millions, MMBTU in Millions, CAD in Millions
Jun. 30, 2017
CAD
MMBTU
MWh
Dec. 31, 2016
CAD
MMBTU
MWh
Commodity contracts:    
Electricity | MWh 5 8
Natural gas | MMBTU 118 107
Foreign currency | CAD CAD 28 CAD 22