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Fair Value Measurements and Financial Instruments - Derivative Instruments (Details)
CAD in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2016
CAD
Dec. 31, 2016
USD ($)
Dec. 31, 2016
CAD
Oct. 14, 2016
USD ($)
Dec. 31, 2015
CAD
Dec. 31, 2014
CAD
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Unrealized losses recognized in regulatory assets     CAD 2,933   CAD 2,532  
Unrealized gains recognized in regulatory liabilities     2,183   1,638  
Cash flow hedges            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Unrealized gain on the derivatives     8   3 CAD 1
Aitken Creek            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Unrealized losses on gas swap contracts CAD 9          
Unrealized losses on gas swap contracts, after tax CAD 6          
UNS Energy            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Realized Gains Shared With Customers, Percent 10.00%          
ITC | Cash flow hedges            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Unrealized gain on the derivatives   $ 8 11      
Derivative instruments            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Unrealized losses recognized in regulatory assets     19   74  
Unrealized gains recognized in regulatory liabilities     CAD 12   CAD 3  
Unsecured | ITC            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Face value | $   385        
Unsecured | ITC            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Face value | $       $ 2,000    
Interest rate swaps | Cash flow hedges            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Loss expected to be reclassified to earnings CAD 5          
Interest rate swaps | Designated as hedging instrument | Cash flow hedges | ITC            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Notional amount | $   $ 100