RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Earnings and Comprehensive Income (Details) - CAD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of unrealized gain/(loss) recognized in OCI	$ 471	$ (698)
Excluded components of fair value hedges	(1)	3
Amount of loss reclassified from AOCI to earnings	65	44
Estimated gain (loss) of AOCI related to cash flow hedges reclassified to earnings in the next 12 months	$ (113)
Maximum term of hedging exposures to variability of cash flows	33 months
Unrealized gain (loss) on derivatives	$ 315	(1,556)
Non-Qualifying Derivative Instruments
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	315	(1,556)
Foreign exchange contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of loss reclassified from AOCI to earnings	1	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	236	(2,003)
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	154	(1,061)
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	82	(942)
Interest rate contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of loss reclassified from AOCI to earnings	63	43
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	2	(18)
Commodity contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of loss reclassified from AOCI to earnings	1	0
Commodity contracts \| Non-Qualifying Derivative Instruments
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	72	473
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	(3)	34
Commodity contracts \| Non-Qualifying Derivative Instruments \| Commodity sales
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	171	1,493
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	(100)	(1,045)
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	4	(9)
Other contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of loss reclassified from AOCI to earnings	0	0
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	(8)
Cash Flow Hedges \| Foreign exchange contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of unrealized gain/(loss) recognized in OCI	(20)	19
Cash Flow Hedges \| Interest rate contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of unrealized gain/(loss) recognized in OCI	497	(715)
Cash Flow Hedges \| Commodity contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of unrealized gain/(loss) recognized in OCI	(8)	9
Cash Flow Hedges \| Other contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of unrealized gain/(loss) recognized in OCI	3	(7)
Fair Value Hedges \| Foreign exchange contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Excluded components of fair value hedges	(1)	3
Net Investment Hedges \| Foreign exchange contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of unrealized gain/(loss) recognized in OCI	$ 0	$ (7)

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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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, after tax and adjustments, of gain (loss) from increase (decrease) in value of excluded component of derivative designated and qualifying as hedge. Adjustments include, but are not limited to, reclassifications for sale and settlement, and amounts recognized under systematic and rational method.

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

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