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Regulatory Assets and Liabilities - Narrative Assets (Details)
CAD in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2016
CAD
Dec. 31, 2015
CAD
Dec. 31, 2016
USD ($)
Dec. 31, 2016
CAD
Regulatory Assets [Line Items]        
Regulatory asset, amortization period 1 year      
Current regulatory assets   CAD 246   CAD 313
Regulatory assets   2,532   2,933
Deferred income taxes (i)        
Regulatory Assets [Line Items]        
Regulatory assets not subject to a regulatory return   351   596
Regulatory assets   936   1,260
Employee future benefits (ii)        
Regulatory Assets [Line Items]        
Regulatory assets not subject to a regulatory return   367   346
Regulatory assets   627   576
Rate stabilization accounts (iii)        
Regulatory Assets [Line Items]        
Regulatory assets not subject to a regulatory return   44   139
Current regulatory assets   49   135
Regulatory assets   119   183
Rate stabilization accounts (iii) | ITC        
Regulatory Assets [Line Items]        
Approved cost-based formula (years) 1 year      
Regulatory asset, amortization period 2 years      
Regional cost allocation recovery for refunds | $     $ 29  
Deferred energy management costs (iv)        
Regulatory Assets [Line Items]        
Regulatory assets not subject to a regulatory return   25   42
Regulatory assets   145   178
Deferred energy management costs (iv) | Minimum        
Regulatory Assets [Line Items]        
Regulatory asset, amortization period 1 year      
Deferred energy management costs (iv) | Maximum        
Regulatory Assets [Line Items]        
Regulatory asset, amortization period 10 years      
Deferred lease costs (vi)        
Regulatory Assets [Line Items]        
Regulatory assets   90   97
Deferred lease costs (vi) | FortisBC Electric        
Regulatory Assets [Line Items]        
Interest expense related to capital lease obligations CAD 31 30    
Depreciation expense related to assets under capital lease 6 6    
Capital lease costs recognized in energy supply costs and operating expenses 7 7    
Deferred lease costs (vi) | FortisBC Electric | Energy supply costs        
Regulatory Assets [Line Items]        
Capital lease costs recognized in energy supply costs and operating expenses 27 26    
Deferred lease costs (vi) | FortisBC Electric | Operating expenses        
Regulatory Assets [Line Items]        
Capital lease costs recognized in energy supply costs and operating expenses CAD 3 3    
Natural gas for transportation incentives (viii)        
Regulatory Assets [Line Items]        
Regulatory asset, amortization period 10 years      
Regulatory assets   25   40
Natural gas for transportation incentives (viii) | FortisBC Energy        
Regulatory Assets [Line Items]        
Regulatory asset, amortization period 10 years      
Derivative instruments (ix)        
Regulatory Assets [Line Items]        
Regulatory assets   74   19
Derivative instruments (ix) | UNS Energy and Central Hudson        
Regulatory Assets [Line Items]        
Regulatory assets not subject to a regulatory return   57   6
Other regulatory assets, individually less than        
Regulatory Assets [Line Items]        
Regulatory assets       40
Approved to be recovered from customers in future rates        
Regulatory Assets [Line Items]        
Regulatory assets   265   296
Other        
Regulatory Assets [Line Items]        
Regulatory assets   329   395
Other regulatory assets not subject to regulatory return        
Regulatory Assets [Line Items]        
Regulatory assets not subject to a regulatory return   CAD 168   CAD 217