RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - EARNINGS AND COMPREHENSIVE INCOME (Details) - CAD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of unrealized gain/(loss) recognized in OCI	$ 121	$ 4
Amount of (gain)/loss reclassified from AOCI to earnings (effective portion)	48	56
Amount of (gain)/loss reclassified from AOCI to earnings (ineffective portion and amount excluded from effectiveness testing)	(1)	2
Estimated gain of AOCI related to cash flow hedges reclassified to earnings in the next 12 months	$ 22
Period to hedge exposures to the variability of cash flows for all forecasted transactions	33 months
Unrealized loss on derivative in fair value hedge	$ 8	2
Unrealized gain on hedged item in fair value hedge	8	2
Total unrealized derivative fair value gain/(loss), net	(260)	418
Gain (loss) on derivative, net	3	0
Non-Qualifying Derivative Instruments
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Total unrealized derivative fair value gain/(loss), net	(260)	418
Gain (loss) on derivative, net	(3)	0
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 (effective portion)	(1)	1
Foreign exchange contracts \| Non-Qualifying Derivative Instruments
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Total unrealized derivative fair value gain/(loss), net	(424)	273
Foreign exchange contracts \| Non-Qualifying Derivative Instruments \| Transportation and other services revenues
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Total unrealized derivative fair value gain/(loss), net	297	159
Foreign exchange contracts \| Non-Qualifying Derivative Instruments \| Other income/(expense)
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Total unrealized derivative fair value gain/(loss), net	(127)	114
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	21	(2)
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	16	8
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 (effective portion)	41	48
Amount of (gain)/loss reclassified from AOCI to earnings (ineffective portion and amount excluded from effectiveness testing)	(1)	2
Interest rate contracts \| Non-Qualifying Derivative Instruments
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Total unrealized derivative fair value gain/(loss), net	(2)	(18)
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	100	(14)
Commodity contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of (gain)/loss reclassified from AOCI to earnings (effective portion)	(1)	(2)
Commodity contracts \| Non-Qualifying Derivative Instruments
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Total unrealized derivative fair value gain/(loss), net	175	163
Commodity contracts \| Non-Qualifying Derivative Instruments \| Transportation and other services revenues
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Total unrealized derivative fair value gain/(loss), net	(1)	(22)
Commodity contracts \| Non-Qualifying Derivative Instruments \| Commodity sales
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Total unrealized derivative fair value gain/(loss), net	82	187
Commodity contracts \| Non-Qualifying Derivative Instruments \| Commodity costs
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Total unrealized derivative fair value gain/(loss), net	84	5
Commodity contracts \| Non-Qualifying Derivative Instruments \| Operating and administrative expense
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Total unrealized derivative fair value gain/(loss), net	10	(7)
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)	21
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 (effective portion)	9	9
Other contracts \| Non-Qualifying Derivative Instruments
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Total unrealized derivative fair value gain/(loss), net	(9)	0
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	$ (14)	$ (9)

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994

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Amount of gain (loss) from the increase (decrease) in fair value of derivative and nonderivative instruments designated as fair value hedging instruments recognized in the income statement.

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Amount of the increase (decrease) in fair value of the hedged item in a fair value hedge recognized in the income statement.

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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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The portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments representing (a) the amount of the hedge ineffectiveness and (b) the amount, if any, excluded from the assessment of hedge effectiveness.

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The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

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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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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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