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Regulatory Assets and Liabilities - Narrative Liabilities (Details)
CAD in Millions, $ in Millions
 1 Months Ended 12 Months Ended
Jun. 30, 2015
Dec. 31, 2013
complaint
Dec. 31, 2016
USD ($)
Dec. 31, 2016
CAD
Dec. 31, 2015
CAD
Regulatory Liabilities [Line Items]          
Current regulatory liabilities       CAD 492 CAD 298
Regulatory liability       2,183 1,638
Employee future benefits (ii)          
Regulatory Liabilities [Line Items]          
Regulatory liability       42 44
Regulatory liabilities not subject to regulatory return       31 36
Rate stabilization accounts (iii)          
Regulatory Liabilities [Line Items]          
Current regulatory liabilities       173 142
Regulatory liability       230 212
Regulatory liabilities not subject to regulatory return       180 123
ROE refund liability (xii)          
Regulatory Liabilities [Line Items]          
Regulatory liability       346 0
ROE refund liability (xii) | ITC          
Regulatory Liabilities [Line Items]          
Complaints (number) | complaint   2      
Regulatory liability | $     $ 258    
ROE refund liability (xii) | ITC | Minimum          
Regulatory Liabilities [Line Items]          
Regulatory liability | $     221    
ROE refund liability (xii) | ITC | Maximum          
Regulatory Liabilities [Line Items]          
Regulatory liability | $     258    
ROE refund liability, initial complaint | ITC | Current regulatory liabilities          
Regulatory Liabilities [Line Items]          
Regulatory liability | $     $ 119    
Electric and gas moderator account (xiii)          
Regulatory Liabilities [Line Items]          
Regulatory liability       71 88
Electric and gas moderator account (xiii) | Central Hudson          
Regulatory Liabilities [Line Items]          
Approved rate (period) 3 years        
Renewable energy surcharge (xiv)          
Regulatory Liabilities [Line Items]          
Regulatory liability       CAD 53 47
Renewable energy surcharge (xiv) | UNS Energy          
Regulatory Liabilities [Line Items]          
Renewable energy target (at least) (percent)     15.00% 15.00%  
Distributed generation Requirement Percentage of Target Percentage (percent)     30.00% 30.00%  
Other regulatory liabilities individually less than          
Regulatory Liabilities [Line Items]          
Regulatory liability       CAD 40  
Approved refund to customers or reduction in future rates          
Regulatory Liabilities [Line Items]          
Regulatory liability       190 156
Other          
Regulatory Liabilities [Line Items]          
Regulatory liability       198 167
Regulatory liabilities not subject to regulatory return       CAD 51 CAD 80