FINANCIAL INSTRUMENTS - Carrying amounts and reconciliation of derivative financial instruments (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure of detailed information about hedging instruments [line items]
Derivative financial asset (liability), net	$ 356	$ 101	$ 489
Current portion of other long-term assets	92	0
Current portion of other long-term liabilities	(17)	(103)
Other long-term assets	281	204
Financial liabilities at amortised cost	23,876	26,299
Cash flow hedges
Disclosure of detailed information about hedging instruments [line items]
Gain (loss) on hedge ineffectiveness	2	5	$ 7
Forward contracts
Disclosure of detailed information about hedging instruments [line items]
Derivatives held for trading, asset (liability)	8	(38)
Forward contracts \| Cash flow hedges
Disclosure of detailed information about hedging instruments [line items]
Cash flow hedges, asset (liability)	70	(71)
WCS differential swaps
Disclosure of detailed information about hedging instruments [line items]
Derivatives held for trading, asset (liability)	(17)	0
AECO basis swap
Disclosure of detailed information about hedging instruments [line items]
Derivatives held for trading, asset (liability)	1	0
AECO fixed price swaps
Disclosure of detailed information about hedging instruments [line items]
Derivatives held for trading, asset (liability)	3	0
Cross currency swaps \| Cash flow hedges
Disclosure of detailed information about hedging instruments [line items]
Cash flow hedges, asset (liability)	$ 291	$ 210

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Derivative Financial Assets (Liabilities) Held For Trading

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Derivative Financial Assets (Liabilities), Net

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Non-Current Derivative Financial Assets (Liabilities)

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The amount of current derivative financial assets. [Refer: Derivative financial assets]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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The amount of current derivative financial liabilities. [Refer: Derivative financial liabilities]

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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.

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The amount of financial liabilities at amortised cost. The amortised cost is the amount at which financial liabilities are measured at initial recognition minus principal repayments, plus or minus the cumulative amortisation using the effective interest method of any difference between that initial amount and the maturity amount. [Refer: Financial liabilities]

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The gain (loss) recognised in profit or loss on hedge ineffectiveness. [Refer: Gain (loss) on hedge ineffectiveness]

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