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Risk Management Activities - Hedging Instruments (Details) $ in Thousands, $ in Thousands, $ in Millions, kJ in Billions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2019 CAD ($) MWh | Dec. 31, 2018 CAD ($) MWh | Dec. 31, 2017 CAD ($) | Dec. 31, 2019 J | Dec. 31, 2019 kJ | Dec. 31, 2019 | Dec. 31, 2019 USD ($) | Dec. 31, 2019 CAD ($) | Dec. 31, 2019 AUD ($) | Dec. 31, 2018 J | Dec. 31, 2018 kJ | Dec. 31, 2018 | Dec. 31, 2018 USD ($) | Dec. 31, 2018 CAD ($) | Dec. 31, 2018 AUD ($)
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collateral paid (Note 15) |  |  |  |  |  |  |  | $ 42,000 |  |  |  |  |  | $ 105,000 |
Notional amount |  |  |  |  |  |  |  | 3,104,000 |  |  |  |  |  | 3,234,000 |
Hedging instrument, assets |  |  |  |  |  |  |  | 7,000 |  |  |  |  |  | 10,000 |
Unrealized pre-tax gains (losses) | $ 1,000 | $ 4,000 | $ 2,000 |  |  |  |  |  |  |  |  |  |  |  |
Borrowings subject to floating interest rates |  |  |  |  |  | 11.00% |  |  |  |  |  | 14.00% |  |  |
Gains (losses) on exchange differences on translation, net of tax | $ (38,000) | $ 43,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Electricity | Non-Hedges | Commodity risk management |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional amount sold | MWh | 16,097,000 | 58,885,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Notional amount purchased | MWh | 7,204,000 | 37,023,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Electricity | Cash flow hedges | Commodity risk management |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional amount sold | MWh | 222,000 | 2,128,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Notional amount purchased | MWh | 0 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt | Net investment hedges |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional amount |  |  |  |  |  |  | $ 370,000 |  |  |  |  |  | $ 400,000 |  |
Long-term debt | Fair value hedges | Interest rate risk |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional amount |  |  |  |  |  |  | 50,000 |  |  |  |  |  | 50,000 |  |
Physical power sales | Cash flow hedges | Commodity risk management |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional amount |  |  |  |  |  |  | 400 |  |  |  |  |  |  |  |
Hedging instrument, assets |  |  |  |  |  |  |  | 678,000 |  |  |  |  |  | 687,000 |
Notional amount purchased | MWh | 23,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) on hedge ineffectiveness | $ 47,000 | $ 60,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Reserve of cash flow hedges |  |  |  |  |  |  |  | 527,000 |  |  |  |  |  | 508,000 |
Foreign exchange forward contracts – foreign-denominated receipts/expenditures | Discontinued Hedge Positions | Currency risk |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Hedging instrument, assets |  |  |  |  |  |  |  |  |  |  |  |  |  | (7,000) |
Hedging instrument, liabilities |  |  |  |  |  |  |  | 0 |  |  |  |  |  | 5,000 |
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 |  |  |  |  |  |  |  | 124,000 |  |  |  |  |  | 0 |
Derivative, Notional Amount Purchased |  |  |  |  |  |  | 95,000 |  |  |  |  |  | 0 |  |
Foreign Exchange Forward Contracts, Foreign-Denominated Debt [Member] | Discontinued Hedge Positions | Currency risk |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional amount sold |  |  |  |  |  |  |  | 191,000 |  |  |  |  |  | 124,000 |
Derivative, Notional Amount Purchased |  |  |  |  |  |  | 150,000 |  |  |  |  |  | 100 |  |
Hedging instrument, assets |  |  |  |  |  |  |  | 6,000 |  |  |  |  |  |  |
Hedging instrument, liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  | (10,000) |
Natural gas | Non-Hedges | Commodity risk management |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional amount sold |  |  |  | 38,062,000 | 80,413 |  |  |  |  | 80,413,000 | 74,194 |  |  |  |
Notional amount purchased |  |  |  | 55,023,000 | 110,488 |  |  |  |  | 110,488,000 | 103,805 |  |  |  |
Transmission | Non-Hedges | Commodity risk management |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional amount sold | MWh | 0 | 29,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Notional amount purchased | MWh | 1,818,000 | 11,163,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Foreign-denominated debt | Net Investment Hedge | Currency risk |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Hedging instrument, assets |  |  |  |  |  |  |  | 483,000 |  |  |  |  |  | 546 |
Gain (loss) on hedge ineffectiveness | $ 21,000 | $ (41,000) |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum | Foreign exchange forward contracts – foreign-denominated receipts/expenditures | Discontinued Hedge Positions | Currency risk |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative, Notional Amount Purchased |  |  |  |  |  |  |  | 139,000 |  |  |  |  |  | 205,000 |
Maximum | Foreign exchange forward contracts – foreign-denominated receipts/expenditures | Discontinued Hedge Positions | Currency risk |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative, Notional Amount Purchased |  |  |  |  |  |  |  | 266,000 |  |  |  |  |  | $ 214,000 |
Hedging instrument, liabilities |  |  |  |  |  |  |  | $ 4,000 |  |  |  |  |  |  |
U.S. Dollars | Foreign exchange forward contracts – foreign-denominated receipts/expenditures | Discontinued Hedge Positions | Currency risk |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional amount sold |  |  |  |  |  |  | $ 108,000 |  |  |  |  |  | $ 164,000 |  |
Australia, Dollars | Foreign exchange forward contracts – foreign-denominated receipts/expenditures | Discontinued Hedge Positions | Currency risk |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notional amount sold |  |  |  |  |  |  |  |  | $ 286 |  |  |  |  |  | $ 218
Reserve of exchange differences on translation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gains (losses) on exchange differences on translation, net of tax | $ 59,000 | $ (84,000) |  |  |  |  |  |  |  |  |  |  |  |  |

X
- Definition The amount of current non-cash collateral assets (such as debt or equity instruments) provided to a transferee, for which the transferee has the right by contract or custom to sell or repledge the collateral. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 9 -IssueDate 2018-01-01 -Paragraph 3.2.23 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=9&date=2018-03-01&anchor=para_3.2.23_a&doctype=Standard -URIDate 2018-03-16 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Note Expiry date 2021-01-01 -Name IAS -Number 39 -IssueDate 2018-01-01 -Paragraph 37 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=39&date=2017-03-02&anchor=para_37_a&doctype=Standard&book=b_pdf -URIDate 2018-03-16 + Details
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- Definition The gain (loss) on hedge ineffectiveness. Hedge ineffectiveness is the extent to which the changes in the fair value or the cash flows of the hedging instrument are greater or less than those on the hedged item. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2018-01-01 -Paragraph 24C -Subparagraph a -Clause i -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2018-03-01&anchor=para_24C_a_i&doctype=Standard -URIDate 2018-03-16 + Details
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