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Risk Management Activities - Net Risk Management Assets and Liabilities (Details) CAD in Millions, $ in Millions | Dec. 31, 2017 CAD | Mar. 31, 2017 CAD | Mar. 31, 2017 USD ($) | Dec. 31, 2016 CAD
--- | --- | --- | --- | ---
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Derivative hedge, notional amount | CAD 690 |  |  |
Financial Assets |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Risk exposure associated with instruments sharing characteristic | 762 |  |  | CAD 920
Not designated as a hedge | Financial Liabilities |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Risk exposure associated with instruments sharing characteristic | 52 |  |  |
Not designated as a hedge | Financial Assets |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Risk exposure associated with instruments sharing characteristic |  |  |  | (16)
Commodity risk management |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Risk exposure associated with instruments sharing characteristic |  |  |  | 744
Commodity risk management | Financial Assets |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Risk exposure associated with instruments sharing characteristic | 728 |  |  |
Commodity risk management | Current |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Risk exposure associated with instruments sharing characteristic | 81 |  |  | 70
Commodity risk management | Long-term |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Risk exposure associated with instruments sharing characteristic | 647 |  |  | 674
Commodity risk management | Not designated as a hedge |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Risk exposure associated with instruments sharing characteristic |  |  |  | (25)
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 | 18 |  |  |
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 |  |  |
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 | 11 |  |  | (9)
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 |  |  |  | (16)
Other |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Risk exposure associated with instruments sharing characteristic |  |  |  | 176
Other | Financial Assets |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Risk exposure associated with instruments sharing characteristic | 34 |  |  |
Other | Current |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Risk exposure associated with instruments sharing characteristic | 37 |  |  | 113
Other | Long-term |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Risk exposure associated with instruments sharing characteristic | (3) |  |  | 63
Other | Not designated as a hedge | Financial Liabilities |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Risk exposure associated with instruments sharing characteristic |  |  |  | 9
Other | Not designated as a hedge | Current |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Risk exposure associated with instruments sharing characteristic |  |  |  | 8
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
Other | Not designated as a hedge | Financial Assets |  |  |  |
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) |  |  |
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 | 710 |  |  | 933
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 |  |  |  | 769
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 | 710 |  |  |
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 | 74 |  |  | 86
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 | 636 |  |  | 683
Cash flow hedges | Other | Designated as a hedge |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Risk exposure associated with instruments sharing characteristic |  |  |  | 164
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 |  |  | 105
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 |  |  | 59
Fair value hedges | Designated as a hedge | Financial Assets |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Risk exposure associated with instruments sharing characteristic | 0 |  |  | 3
Fair value hedges | Commodity risk management | Designated as a hedge |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Risk exposure associated with instruments sharing characteristic |  |  |  | 0
Fair value 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 | 0 |  |  |
Fair value 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 | 0 |  |  | 0
Fair value 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 | 0 |  |  | 0
Fair value hedges | Other | Designated as a hedge |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Risk exposure associated with instruments sharing characteristic |  |  |  | 3
Fair value 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 |  |  |
Fair value 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
Fair value hedges | Other | Designated as a hedge | Long-term |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Risk exposure associated with instruments sharing characteristic | CAD 0 |  |  | CAD 3
Long-term debt | Discontinued Cash Flow Hedge Position [Member] | Interest rate risk |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Derivative hedge, notional amount | $ |  |  | $ 690 |
Long-term debt | Discontinued Hedge Positions [Member] | Interest rate risk |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Derivative hedge, notional amount |  | CAD 50 |  |

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