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RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Earnings and Comprehensive Income (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 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	$ (303)
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI		$ 20	$ (608)	$ 141
Amount of (gain) loss reclassified from AOCI to earnings	46
Amount of (gain) loss reclassified from AOCI to earnings		46	71	93
Estimated gain (loss) of AOCI related to cash flow hedges reclassified to earnings in the next 12 months			$ (70)
Period to hedge exposures to the variability of cash flows for all forecasted transactions			30 months
Unrealized gain/(loss) on derivative	0	(4)	$ 0	3
Unrealized gain/(loss) on hedged item	0	3	0	(3)
Realized gain/(loss) on derivative	0	2	0	(1)
Realized gain/(loss) on hedged item	0	(2)	0	1
Unrealized gain (loss) on derivatives			1,112	(549)
Non-Qualifying Derivative Instruments
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	574	(289)	1,112	(549)
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	0
Amount of (gain) loss reclassified from AOCI to earnings		(2)	2	(3)
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	412	(277)	1,028	(701)
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			550	(555)
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			478	(146)
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	(3)
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI		(3)	(13)	18
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		(5)	2	11
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	40
Amount of (gain) loss reclassified from AOCI to earnings		54	72	94
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	0	0	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	(285)
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI		17	(581)	117
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		0	0	(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	162	(19)	(99)	156
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			(25)	3
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			(490)	10
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			392	127
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			24	16
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	(18)
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI		(1)	(21)	(3)
Other contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of (gain) loss reclassified from AOCI to earnings	6
Amount of (gain) loss reclassified from AOCI to earnings		(6)	(3)	3
Other contracts \| Non-Qualifying Derivative Instruments
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	0	7	5	(2)
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	$ 2
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI		$ 12	$ 5	$ (2)