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Risk Management Activities - Net Risk Management Assets and Liabilities (Details) - CAD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Financial Assets |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | $ 677 | $ 762
Not designated as a hedge | Financial Liabilities |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | (10) |
Not designated as a hedge | Financial Assets |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic |  | 52
Commodity risk management |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic |  | 728
Commodity risk management | Financial Assets |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | 679 |
Commodity risk management | Current |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | 59 | 81
Commodity risk management | Long-term |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | 620 | 647
Commodity risk management | Not designated as a hedge |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic |  | 18
Commodity risk management | Not designated as a hedge | Financial Liabilities |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | (8) |
Commodity risk management | Not designated as a hedge | Current |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | 0 |
Commodity risk management | Not designated as a hedge | Long-term |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | (8) | 11
Commodity risk management | Not designated as a hedge | Current |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic |  | 7
Other |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic |  | 34
Other | Financial Assets |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | (2) |
Other | Current |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | (3) | 37
Other | Long-term |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | 1 | (3)
Other | Not designated as a hedge | Financial Liabilities |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic |  | 34
Other | Not designated as a hedge | Current |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic |  | 37
Other | Not designated as a hedge | Long-term |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic |  | (3)
Other | Not designated as a hedge | Financial Assets |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | (2) |
Other | Not designated as a hedge | Current |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | (3) |
Other | Not designated as a hedge | Long-term |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | 1 |
Cash flow hedges | Designated as a hedge | Financial Assets |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | 687 | 710
Cash flow hedges | Commodity risk management | Designated as a hedge |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic |  | 710
Cash flow hedges | Commodity risk management | Designated as a hedge | Financial Assets |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | 687 |
Cash flow hedges | Commodity risk management | Designated as a hedge | Current |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | 59 | 74
Cash flow hedges | Commodity risk management | Designated as a hedge | Long-term |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | 628 | 636
Cash flow hedges | Other | Designated as a hedge |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic |  | 0
Cash flow hedges | Other | Designated as a hedge | Financial Assets |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | 0 |
Cash flow hedges | Other | Designated as a hedge | Current |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | 0 | 0
Cash flow hedges | Other | Designated as a hedge | Long-term |  |
Disclosure of detailed information about financial instruments [line items] |  |
Risk exposure associated with instruments sharing characteristic | $ 0 | $ 0

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