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Risk Management Activities - Effect on Cash Flow Hedges (Details) - CAD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Disclosure of detailed information about financial instruments [line items] |  |  |
Gains (losses) on cash flow hedges, before tax | $ 77 | $ (9) | $ 136
Other comprehensive income that will be reclassified to profit or loss, net of tax | (19) | (11) | 82
Gain (loss) on hedge ineffectiveness recognised in other comprehensive income | (53) | (57) | (138)
Gain (loss) on hedge ineffectiveness recognised in profit or loss | 0 | 0 | 0
Accumulated other comprehensive income (Note 26) | 454 | 481 |
Reserve of exchange differences on translation |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Accumulated other comprehensive income (Note 26) | (21) | 17 | (26)
Commodity contracts |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Gains (losses) on cash flow hedges, before tax | 77 | (9) | 163
Foreign exchange forwards on project hedges |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Gains (losses) on cash flow hedges, before tax |  |  | (1)
Foreign exchange forwards on US debt |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Gains (losses) on cash flow hedges, before tax |  | 0 | 0
Cross-currency swaps |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Gains (losses) on cash flow hedges, before tax |  |  | (26)
Forward starting interest rate swaps |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Gains (losses) on cash flow hedges, before tax | 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 | (59) | (67) | (172)
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
Gain (loss) on hedge ineffectiveness recognised in profit or loss |  |  | 0
Profit (Loss) |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Gain (loss) on hedge ineffectiveness recognised in profit or loss |  |  | 31
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 | 6 | 7 | 7
Gain (loss) on hedge ineffectiveness recognised in profit or loss | 0 | 0 | 0
Foreign exchange (gain) loss |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
Gain (loss) on hedge ineffectiveness recognised in profit or loss |  |  | 15
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
Gain (loss) on hedge ineffectiveness recognised in profit or loss |  | $ 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
Gain (loss) on hedge ineffectiveness recognised in profit or loss |  |  | $ 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 |  |

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