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Risk Management Activities - Non-Hedges (Details) t in Thousands, $ in Millions, kJ in Billions | 12 Months Ended
--- | ---
Dec. 31, 2018 CAD ($) MWh kJ t | Dec. 31, 2017 CAD ($) MWh kJ t | Dec. 31, 2016 CAD ($)
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Foreign exchange gain (loss) | $ (15) | $ (1) | $ (5)
Gains (losses) on change in fair value of derivatives | 29 | 45 | 63
Gains on change in fair value of derivatives | 3 | 28 | 9
Unrealised Loss On Change In Fair Value Of Derivatives | 4 | (2) | 4
Unrealised Gains On Change In Fair Value Of Derivatives | $ (1) | $ 30 | $ 5
Electricity | Non-Hedges | Commodity risk management |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Notional amount sold | MWh | 58,885,000 | 14,688,000 |
Notional amount purchased | MWh | 37,023,000 | 7,348,000 |
Natural gas | Non-Hedges | Commodity risk management |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Notional amount sold | kJ | 80,413 | 74,194 |
Notional amount purchased | kJ | 110,488 | 103,805 |
Transmission | Non-Hedges | Commodity risk management |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Notional amount sold | MWh | 29,000 | 1,000 |
Notional amount purchased | MWh | 11,163,000 | 3,455,000 |
Emissions | Non-Hedges | Commodity risk management |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Derivative, Nonmonetary Notional Amount Sold, Emissions | t | 3,134 | 516 |
Derivative, Nonmonetary Notional Amount Purchased, Emissions | t | 2,948 | 717 |

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