RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - EFFECTS ON EARNINGS AND COMPREHENSIVE INCOME (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of unrealized gain/(loss) recognized in OCI	$ (579)	$ (591)	$ (141)
Amount (gain)/loss reclassified from AOCI to earnings	$ 256	158	191
Period to hedge exposures to the variability of cash flows for all forecasted transactions	36 months
Unrealized gain (loss) on derivatives	$ 756	1,751	(903)
Realized loss on derivative	265	346	(1,100)
Cash Flow Hedges
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Estimated loss of AOCI related to cash flow hedges reclassified to earnings in the next 12 months	127
Foreign exchange contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount (gain)/loss reclassified from AOCI to earnings	5	5	5
Foreign exchange contracts \| Cash Flow Hedges
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of unrealized gain/(loss) recognized in OCI	(1)	(19)	19
Foreign exchange contracts \| Fair Value Hedges
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of unrealized gain/(loss) recognized in OCI	5	0	0
Foreign exchange contracts \| Net Investment Hedges
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of unrealized gain/(loss) recognized in OCI	13	2	31
Interest rate contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount (gain)/loss reclassified from AOCI to earnings	253	157	184
Interest rate contracts \| Cash Flow Hedges
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of unrealized gain/(loss) recognized in OCI	(595)	(559)	(190)
Interest rate contracts \| Fair Value Hedges
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	(116)	0
Unrealized gain on hedged item	133	0
Realized loss on derivative	(12)	0
Realized loss on hedged item	0	0
Commodity contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount (gain)/loss reclassified from AOCI to earnings	0	(1)	(1)
Commodity contracts \| Cash Flow Hedges
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of unrealized gain/(loss) recognized in OCI	2	(25)	2
Other contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount (gain)/loss reclassified from AOCI to earnings	(2)	(3)	3
Other contracts \| Cash Flow Hedges
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of unrealized gain/(loss) recognized in OCI	(3)	10	(3)
Non-Qualifying Derivative Instruments
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	756	1,751	(903)
Non-Qualifying Derivative Instruments \| Foreign exchange contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	902	1,626	(1,390)
Non-Qualifying Derivative Instruments \| Foreign exchange contracts \| Transportation and other services
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	533	930	(1,108)
Non-Qualifying Derivative Instruments \| Foreign exchange contracts \| Other income/(expense)
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	369	696	(282)
Non-Qualifying Derivative Instruments \| Interest rate contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	(25)	178	5
Non-Qualifying Derivative Instruments \| Commodity contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	(114)	(62)	485
Non-Qualifying Derivative Instruments \| Commodity contracts \| Transportation and other services
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	(2)	(26)	66
Non-Qualifying Derivative Instruments \| Commodity contracts \| Commodity sales / costs
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	(321)	(544)	599
Non-Qualifying Derivative Instruments \| Commodity contracts \| Commodity Costs
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	207	459	(193)
Non-Qualifying Derivative Instruments \| Commodity contracts \| Operating and administrative expense
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	2	49	13
Non-Qualifying Derivative Instruments \| Other contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	$ (7)	$ 9	$ (3)

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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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 of unrealized gain (loss) related to derivatives designated as cash flow hedging instruments, recorded in results of operations to the extent that the cash flow hedge is determined to be ineffective.

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