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Dispositions - Non-Regulated Generation Assets (Details)
CAD in Millions, $ in Millions
1 Months Ended 12 Months Ended
Jul. 31, 2015
CAD
Jun. 30, 2015
USD ($)
Jun. 30, 2015
CAD
Dec. 31, 2016
CAD
Dec. 31, 2015
CAD
Long Lived Assets Held-for-sale [Line Items]          
Gain (loss) on sale of non-regulated generation assets, net of expenses       CAD 0 CAD 56
Upstate New York, Non-Regulated Generating Assets          
Long Lived Assets Held-for-sale [Line Items]          
Proceeds from the sale of non-regulated generation assets   $ 63 CAD 77    
Gain (loss) on sale of non-regulated generation assets, net of expenses   41 51    
Gain (loss) on sale of non-regulated generation assets, net of expense and tax   $ 22 CAD 27    
Ontario, Non-Regulated Generating Assets [Member]          
Long Lived Assets Held-for-sale [Line Items]          
Proceeds from the sale of non-regulated generation assets CAD 16        
Gain (loss) on sale of non-regulated generation assets, net of expenses 5        
Gain (loss) on sale of non-regulated generation assets, net of expense and tax CAD 5        
New York and Ontario, Non-Regulated Generating Assets          
Long Lived Assets Held-for-sale [Line Items]          
Earnings before taxes related to disposal, excluding gain of sale         CAD 1