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Risk Management Activities - Effect on Cash Flow Hedges (Details) - CAD
CAD in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of detailed information about financial instruments [line items]      
Total gains included in earnings before income taxes CAD 136 CAD 0 CAD 517
Other comprehensive income that will be reclassified to profit or loss, net of tax 82 78 252
Gain (loss) on hedge ineffectiveness recognised in other comprehensive income (138) 0 (250)
Gain (loss) on hedge ineffectiveness recognised in profit or loss 0 0 5
Commodity contracts      
Disclosure of detailed information about financial instruments [line items]      
Total gains included in earnings before income taxes 163 0 308
Foreign exchange forwards on commodity contracts      
Disclosure of detailed information about financial instruments [line items]      
Total gains included in earnings before income taxes 0 0 32
Foreign exchange forwards on project hedges      
Disclosure of detailed information about financial instruments [line items]      
Total gains included in earnings before income taxes (1) 0 4
Foreign exchange forwards on US debt      
Disclosure of detailed information about financial instruments [line items]      
Total gains included in earnings before income taxes 0 0 10
Cross-currency swaps      
Disclosure of detailed information about financial instruments [line items]      
Total gains included in earnings before income taxes (26) 0 163
Forward starting interest rate swaps      
Disclosure of detailed information about financial instruments [line items]      
Total gains included in earnings before income taxes 0 0 0
Revenue | Commodity contracts      
Disclosure of detailed information about financial instruments [line items]      
Gain (loss) on hedge ineffectiveness recognised in other comprehensive income (172) 0 (110)
Gain (loss) on hedge ineffectiveness recognised in profit or loss 0 0 5
Revenue | Foreign exchange forwards on commodity contracts      
Disclosure of detailed information about financial instruments [line items]      
Gain (loss) on hedge ineffectiveness recognised in other comprehensive income 0 0 (12)
Gain (loss) on hedge ineffectiveness recognised in profit or loss 0 0 0
Property, plant, and equipment | Foreign exchange forwards on project hedges      
Disclosure of detailed information about financial instruments [line items]      
Gain (loss) on hedge ineffectiveness recognised in other comprehensive income 0 0 (1)
Gain (loss) on hedge ineffectiveness recognised in profit or loss 0 0 0
Profit (Loss)      
Disclosure of detailed information about financial instruments [line items]      
Gain (loss) on hedge ineffectiveness recognised in profit or loss     6
Fuel and purchased power | Commodity contracts      
Disclosure of detailed information about financial instruments [line items]      
Gain (loss) on hedge ineffectiveness recognised in other comprehensive income 0 0 41
Gain (loss) on hedge ineffectiveness recognised in profit or loss 0 0 0
Foreign exchange (gain) loss | Foreign exchange forwards on US debt      
Disclosure of detailed information about financial instruments [line items]      
Gain (loss) on hedge ineffectiveness recognised in other comprehensive income 3 0 (12)
Gain (loss) on hedge ineffectiveness recognised in profit or loss 0 0 0
Foreign exchange (gain) loss | Cross-currency swaps      
Disclosure of detailed information about financial instruments [line items]      
Gain (loss) on hedge ineffectiveness recognised in other comprehensive income 24 0 (163)
Gain (loss) on hedge ineffectiveness recognised in profit or loss 0 0 0
Interest expense | Forward starting interest rate swaps      
Disclosure of detailed information about financial instruments [line items]      
Gain (loss) on hedge ineffectiveness recognised in other comprehensive income 7 0 7
Gain (loss) on hedge ineffectiveness recognised in profit or loss 0 0 CAD 0
Mississauga Congeneration Facility [Member]      
Disclosure of detailed information about financial instruments [line items]      
Total gains included in earnings before income taxes   31  
Gains (losses) on exchange differences on translation, before tax   CAD 15  
Scenario, Forecast      
Disclosure of detailed information about financial instruments [line items]      
Other comprehensive income that will be reclassified to profit or loss, net of tax CAD 85