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Risk Management Activities - Hedging Instruments (Details) CAD in Thousands, € in Millions, ¥ in Millions, AUD in Millions, $ in Millions, kJ in Billions | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Mar. 31, 2017 CAD | Dec. 31, 2017 CAD MWh kJ | Dec. 31, 2016 CAD MWh kJ | Dec. 31, 2015 CAD | Dec. 31, 2017 USD ($) MWh kJ | Dec. 31, 2017 JPY (¥) MWh kJ | Dec. 31, 2017 AUD MWh kJ | Dec. 31, 2017 EUR (€) MWh kJ | Mar. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) MWh kJ | Dec. 31, 2016 JPY (¥) MWh kJ | Dec. 31, 2016 AUD MWh kJ
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Derivative hedge, notional amount |  | CAD 690,000 |  |  |  |  |  |  |  |  |  |
Collateral paid (Note 14) |  | CAD 67,000 | CAD 77,000 |  |  |  |  |  |  |  |  |
Interest rate |  | 6.65% | 6.65% |  | 6.65% | 6.65% | 6.65% | 6.65% |  | 6.65% | 6.65% | 6.65%
Notional amount |  | CAD 3,664,000 | CAD 4,311,000 |  |  |  |  |  |  |  |  |
Fair value asset |  | 30,000 | 163,000 |  |  |  |  |  |  |  |  |
Unrealized pre-tax gains (losses) |  | 2,000 | 0 | CAD 3,000 |  |  |  |  |  |  |  |
Cumulative gains (loss) in derivative instruments |  | CAD 1,000 | CAD 4,000 |  |  |  |  |  |  |  |  |
Borrowings subject to floating interest rates |  | 4.70% | 6.00% | 6.00% | 4.70% | 4.70% | 4.70% | 4.70% |  | 6.00% | 6.00% | 6.00%
Cash flow hedges | Currency risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Fair value asset |  |  | CAD 0 |  |  |  |  |  |  |  |  |
Long-term debt | Discontinued Hedge Positions [Member] | Interest rate risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Derivative hedge, notional amount | CAD 50,000 |  |  |  |  |  |  |  |  |  |  |
Fair value asset |  | CAD 1,000 |  |  |  |  |  |  |  |  |  |
Fair value hedge, cumulative gain (loss) amortized | CAD 2,000 |  |  |  |  |  |  |  |  |  |  |
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 | Net investment hedges |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Notional amount | $ |  |  |  |  | $ 480 |  |  |  |  | $ 630 |  |
Long-term debt | Fair value hedges | Interest rate risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Notional amount | $ |  |  |  |  | 50 |  |  |  |  | 50 |  |
Fair value asset |  | 0 | 3,000 |  |  |  |  |  |  |  |  |
Foreign Currency Forward Contracts | Cash flow hedges | Currency risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Fair value asset |  |  | 0 |  |  |  |  |  |  |  |  |
Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures CAD 1 | Cash flow hedges | Currency risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Notional amount sold |  | 9,000 |  |  |  |  |  |  |  |  |  |
Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures CAD 2 | Cash flow hedges | Currency risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Notional amount sold |  | 14,000 |  |  |  |  |  |  |  |  |  |
Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures | Discontinued Hedge Positions [Member] | Currency risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Fair value asset |  | 11,000 | 12,000 |  |  |  |  |  |  |  |  |
Fair value liability |  | 9,000 | 3,000 |  |  |  |  |  |  |  |  |
Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures | Cash flow hedges | Currency risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Notional amount sold | AUD |  |  |  |  |  |  | AUD 1 |  |  |  |  | AUD 8
Notional amount purchased |  |  |  |  | 7 | ¥ 119 |  | € 9 |  |  | ¥ 710 |
Fair value asset |  | 0 |  |  |  |  |  |  |  |  |  |
Foreign Exchange Forward Contracts - foreign-denominated debt | Discontinued Hedge Positions [Member] | Currency risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Notional amount sold |  | 294,000 |  |  |  |  |  |  |  |  |  |
Notional amount purchased | $ |  |  |  |  | 230 |  |  |  |  |  |  |
Fair value liability |  | 4,000 |  |  |  |  |  |  |  |  |  |
Foreign Exchange Forward Contracts - foreign-denominated debt | Cash flow hedges | Currency risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Notional amount sold |  |  | 26,000 |  |  |  |  |  |  |  |  |
Notional amount purchased | $ |  |  |  |  |  |  |  |  |  | 20 |  |
Fair value asset |  | 0 |  |  |  |  |  |  |  |  |  |
Cross-Currency Swaps - foreign-denominated debt | Discontinued Hedge Positions [Member] | Currency risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Notional amount sold |  | 306,000 |  |  |  |  |  |  |  |  |  |
Notional amount purchased | $ |  |  |  |  | $ 270 |  |  |  |  |  |  |
Fair value liability |  | (35,000) |  |  |  |  |  |  |  |  |  |
Cross-Currency Swaps - foreign-denominated debt | Cash flow hedges | Currency risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Notional amount sold |  |  | 434,000 |  |  |  |  |  |  |  |  |
Notional amount purchased | $ |  |  |  |  |  |  |  |  |  | 400 |  |
Fair value asset |  | CAD 0 |  |  |  |  |  |  |  |  |  |
Cross-Currency Swaps - foreign-denominated debt 2 | Cash flow hedges | Currency risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Notional amount sold |  |  | CAD 306,000 |  |  |  |  |  |  |  |  |
Notional amount purchased | $ |  |  |  |  |  |  |  |  |  | $ 270 |  |
Natural gas | Non-Hedges [Member] | Commodity risk management |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Notional amount sold | kJ |  | 74,194 | 146,113 |  | 74,194 | 74,194 | 74,194 | 74,194 |  | 146,113 | 146,113 | 146,113
Notional amount purchased | kJ |  | 103,805 | 173,187 |  | 103,805 | 103,805 | 103,805 | 103,805 |  | 173,187 | 173,187 | 173,187
Transmission | Non-Hedges [Member] | Commodity risk management |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Notional amount sold | MWh |  | 1,000 | 0 |  | 1,000 | 1,000 | 1,000 | 1,000 |  | 0 | 0 | 0
Notional amount purchased | MWh |  | 3,455,000 | 3,429,000 |  | 3,455,000 | 3,455,000 | 3,455,000 | 3,455,000 |  | 3,429,000 | 3,429,000 | 3,429,000
2019 | Cross-Currency Swaps - foreign-denominated debt | Cash flow hedges | Currency risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Fair value asset |  |  | CAD 59,000 |  |  |  |  |  |  |  |  |
2018 | Cross-Currency Swaps - foreign-denominated debt | Cash flow hedges | Currency risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Fair value asset |  |  | 104,000 |  |  |  |  |  |  |  |  |
Minimum | Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures | Discontinued Hedge Positions [Member] | Currency risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Notional amount purchased |  | CAD 104,000 | 216,000 |  |  |  |  |  |  |  |  |
Maximum | Foreign Currency Forward Contracts | Cash flow hedges | Currency risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Fair value asset |  |  | 1,000 |  |  |  |  |  |  |  |  |
Maximum | Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures | Discontinued Hedge Positions [Member] | Currency risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Notional amount purchased |  | CAD 157,000 | CAD 219,000 |  |  |  |  |  |  |  |  |
U.S. Dollars | Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures | Discontinued Hedge Positions [Member] | Currency risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Notional amount sold | $ |  |  |  |  | $ 73 |  |  |  |  | $ 152 |  |
Australia, Dollars | Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures | Discontinued Hedge Positions [Member] | Currency risk |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |
Notional amount sold | AUD |  |  |  |  |  |  | AUD 170 |  |  |  |  | AUD 232

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