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Property, plant and equipment (Details) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Property, Plant and Equipment, Net, by Type [Abstract]    
Total cost $ 24,197 $ 23,673
Less: Accumulated depreciation (7,824) (7,787)
Property, Plant and Equipment, Net 16,995 17,290
Net Book Value 16,373 15,886
Electric Generation Equipment [Member]    
Property, Plant and Equipment, Net, by Type [Abstract]    
Total cost $ 11,010 10,553
Electric Generation Equipment [Member] | Minimum [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Useful Life 3 years  
Electric Generation Equipment [Member] | Maximum [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Useful Life 131 years  
Electric Transmission [Member]    
Property, Plant and Equipment, Net, by Type [Abstract]    
Total cost $ 2,786 2,799
Electric Transmission [Member] | Minimum [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Useful Life 15 years  
Electric Transmission [Member] | Maximum [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Useful Life 80 years  
Electric Distribution [Member]    
Property, Plant and Equipment, Net, by Type [Abstract]    
Total cost $ 5,660 5,715
Electric Distribution [Member] | Minimum [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Useful Life 4 years  
Electric Distribution [Member] | Maximum [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Useful Life 80 years  
Gas, Transmission and Distribution Equipment [Member]    
Property, Plant and Equipment, Net, by Type [Abstract]    
Total cost $ 2,867 2,895
Gas, Transmission and Distribution Equipment [Member] | Minimum [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Useful Life 7 years  
Gas, Transmission and Distribution Equipment [Member] | Maximum [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Useful Life 85 years  
Other Energy Equipment [Member]    
Property, Plant and Equipment, Net, by Type [Abstract]    
Total cost $ 1,874 1,711
Other Energy Equipment [Member] | Minimum [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Useful Life 3 years  
Other Energy Equipment [Member] | Maximum [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Useful Life 60 years  
Construction in Progress [Member]    
Property, Plant and Equipment, Net, by Type [Abstract]    
Total cost $ 622 $ 1,404