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Financial Instruments - Carrying Amounts and Reconciliation of Derivative Financial Instruments (Details) - CAD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Disclosure of detailed information about hedging instruments [line items] |  |  |
Derivative financial asset (liability), net | $ 178 | $ 356 | $ 101
Current portion of other long-term assets | 8 | 92 |
Current portion of other long-term liabilities | (112) | (17) |
Other long-term assets | 282 | 281 |
Cash flow hedges |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Gain (loss) on hedge ineffectiveness | 3 | 2 | $ 5
Forward contracts |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Derivatives held for trading, asset (liability) | (10) | 8 |
Forward contracts | Cash flow hedges |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Cash flow hedges, asset (liability) | (91) | 70 |
AECO basis swap |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Derivatives held for trading, asset (liability) | (8) | 1 |
AECO fixed price swaps |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Derivatives held for trading, asset (liability) | (3) | 3 |
WCS differential swaps |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Derivatives held for trading, asset (liability) | 0 | (17) |
Cross currency swaps | Cash flow hedges |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Cash flow hedges, asset (liability) | $ 290 | $ 291 |

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- Definition Derivative Financial Assets (Liabilities) Held For Hedging + References No definition available. + Details
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- Definition Derivative Financial Assets (Liabilities) Held For Trading + References No definition available. + Details
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- Definition Derivative Financial Assets (Liabilities), Net + References No definition available. + Details
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- Definition Non-Current Derivative Financial Assets (Liabilities) + References No definition available. + Details
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- Definition The amount of current derivative financial assets. [Refer: Derivative financial assets] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2019-01-01 -Paragraph 55 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_55&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition The amount of current derivative financial liabilities. [Refer: Derivative financial liabilities] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2019-01-01 -Paragraph 55 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_55&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table. + References No definition available. + Details
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- Definition The gain (loss) recognised in profit or loss on hedge ineffectiveness. [Refer: Gain (loss) on hedge ineffectiveness] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2019-01-01 -Paragraph 24C -Subparagraph a -Clause i -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2019-en-r&anchor=para_24C_a_i&doctype=Standard -URIDate 2019-03-27 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2019-01-01 -Paragraph 24C -Subparagraph b -Clause ii -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2019-en-r&anchor=para_24C_b_ii&doctype=Standard -URIDate 2019-03-27 + Details
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