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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, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of detailed information about hedging instruments [line items] |  |  |
Derivative financial asset (liability), net | $ 101 | $ 489 | $ 854
Current portion of other long-term (liabilities) assets | (103) | 222 |
Other long-term assets | 204 | 267 |
Cash flow hedges |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Gain (loss) on hedge ineffectiveness | 5 | 7 | $ 5
Forward contracts |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Derivatives held for trading, asset (liability) | (38) | 10 |
Forward contracts | Cash flow hedges |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Cash flow hedges, asset (liability) | (71) | 16 |
AECO swaps |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Derivatives held for trading, asset (liability) | 0 | (6) |
Cross currency swaps | Cash flow hedges |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Cash flow hedges, asset (liability) | $ 210 | $ 469 |

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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), Net + 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 -Note Effective 2018-01-01 -Name IFRS -Number 7 -IssueDate 2017-01-01 -Paragraph 24C -Subparagraph a -Clause i -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-01&anchor=para_24C_a_i&doctype=Standard -URIDate 2017-03-09 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Note Effective 2018-01-01 -Name IFRS -Number 7 -IssueDate 2017-01-01 -Paragraph 24C -Subparagraph b -Clause ii -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-01&anchor=para_24C_b_ii&doctype=Standard -URIDate 2017-03-09 + Details
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