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Risk Management Activities - Effect on Cash Flow Hedges (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of detailed information about financial instruments [line items]      
Total gains included in earnings before income taxes $ (9) $ 136 $ 271
Other comprehensive income that will be reclassified to profit or loss, net of tax (11) 82 78
Gain (loss) on hedge ineffectiveness recognised in other comprehensive income (57) (138) (105)
Gain (loss) on hedge ineffectiveness recognised in profit or loss 0 0 16
Commodity contracts      
Disclosure of detailed information about financial instruments [line items]      
Total gains included in earnings before income taxes (9) 163 304
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 (5)
Foreign exchange forwards on project hedges      
Disclosure of detailed information about financial instruments [line items]      
Total gains included in earnings before income taxes 0 (1) (1)
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 (2)
Cross-currency swaps      
Disclosure of detailed information about financial instruments [line items]      
Total gains included in earnings before income taxes 0 (26) (25)
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 (67) (172) (169)
Gain (loss) on hedge ineffectiveness recognised in profit or loss 0 0 0
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 (16)
Gain (loss) on hedge ineffectiveness recognised in profit or loss 0 0 (15)
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 0
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     31
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 44
Gain (loss) on hedge ineffectiveness recognised in profit or loss 0 0 31
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 3 53
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 0 24 (23)
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 7 6
Gain (loss) on hedge ineffectiveness recognised in profit or loss 0 $ 0 $ 0
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 $ 68