RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Earnings and Comprehensive Income (Details) - CAD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI	$ (296)
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI		$ 121
Amount of (gain) loss reclassified from AOCI to earnings	25
Amount of (gain) loss reclassified from AOCI to earnings		47
Estimated gain of AOCI related to cash flow hedges reclassified to earnings in the next 12 months	$ 36
Period to hedge exposures to the variability of cash flows for all forecasted transactions	33 months
Unrealized gain/(loss) on derivative		(8)
Unrealized gain/(loss) on hedged item		8
Realized gain/(loss) on derivative		(3)
Realized gain/(loss) on hedged item		3
Unrealized gain (loss) on derivatives	$ 538	(260)
Non-Qualifying Derivative Instruments
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	538	(260)
Foreign exchange contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of (gain) loss reclassified from AOCI to earnings	2
Amount of (gain) loss reclassified from AOCI to earnings		(1)
Foreign exchange contracts \| Non-Qualifying Derivative Instruments
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	616	(424)
Foreign exchange contracts \| Non-Qualifying Derivative Instruments \| Transportation and other services
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	352	(297)
Foreign exchange contracts \| Non-Qualifying Derivative Instruments \| Other income/(expense)
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	264	(127)
Foreign exchange contracts \| Cash Flow Hedges
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI	(10)
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI		21
Foreign exchange contracts \| Net Investment Hedges
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI	1
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI		16
Interest rate contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of (gain) loss reclassified from AOCI to earnings	32
Interest rate contracts \| Interest expense
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of (gain) loss reclassified from AOCI to earnings	32
Amount of (gain) loss reclassified from AOCI to earnings		40
Interest rate contracts \| Non-Qualifying Derivative Instruments
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	178	(2)
Interest rate contracts \| Cash Flow Hedges
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI	(296)
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI		100
Commodity contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of (gain) loss reclassified from AOCI to earnings	0
Amount of (gain) loss reclassified from AOCI to earnings		(1)
Commodity contracts \| Non-Qualifying Derivative Instruments
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	(261)	175
Commodity contracts \| Non-Qualifying Derivative Instruments \| Transportation and other services
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	(26)	(1)
Commodity contracts \| Non-Qualifying Derivative Instruments \| Commodity
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	(642)	82
Commodity contracts \| Non-Qualifying Derivative Instruments \| Commodity costs
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	398	84
Commodity contracts \| Non-Qualifying Derivative Instruments \| Operating and administrative expense
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	9	10
Commodity contracts \| Cash Flow Hedges
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI	(3)
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI		(2)
Other contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of (gain) loss reclassified from AOCI to earnings	(9)
Other contracts \| Operating and administrative expense
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of (gain) loss reclassified from AOCI to earnings	(9)
Amount of (gain) loss reclassified from AOCI to earnings		9
Other contracts \| Non-Qualifying Derivative Instruments
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	5	(9)
Other contracts \| Cash Flow Hedges
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI	$ 12
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI		$ (14)

The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.

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Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

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The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

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Amount of gain (loss) from the ineffectiveness of the hedge.

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Gain (loss) incurred in hedging activities.

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Maximum length of time over which the entity is hedging its exposure to the variability in future cash flows for forecasted transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount, before tax and reclassification, of gain (loss) from derivative instrument designated and qualifying cash flow hedge included in assessment of hedge effectiveness.

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Amount, before tax, of reclassification of gain (loss) from accumulated other comprehensive income (AOCI) for derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

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Amount before tax of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

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