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Goodwill (Details) - CAD
CAD in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Goodwill [Roll Forward]    
Balance, beginning of year CAD 4,173 CAD 3,732
Foreign currency translation impacts 58 441
Balance, end of year 12,364 4,173
ITC    
Goodwill [Roll Forward]    
Acquisition 8,106 0
Aitken Creek    
Goodwill [Roll Forward]    
Acquisition CAD 27 CAD 0