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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 |  |

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