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RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Earnings and Comprehensive Income (Details) - CAD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 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	$ (230,000,000)		$ (838,000,000)
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI		$ 72,000,000		$ 213,000,000
Amount of (gain) loss reclassified from AOCI to earnings	37,000,000		108,000,000
Amount of (gain) loss reclassified from AOCI to earnings		52,000,000		145,000,000
Estimated gain (loss) of AOCI related to cash flow hedges reclassified to earnings in the next 12 months			$ (72,000,000)
Period to hedge exposures to the variability of cash flows for all forecasted transactions			27 months
Unrealized gain/(loss) on derivative		3,000,000		(9,000,000)
Unrealized gain/(loss) on hedged item		(3,000,000)		8,000,000
Realized gain/(loss) on derivative		(3,000,000)		(4,000,000)
Realized gain/(loss) on hedged item		3,000,000		4,000,000
Outstanding fair value hedges	0		$ 0
Unrealized gain (loss) on derivatives			1,005,000,000	(319,000,000)
Non-Qualifying Derivative Instruments
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	(107,000,000)	230,000,000	1,005,000,000	(319,000,000)
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	2,000,000		4,000,000
Amount of (gain) loss reclassified from AOCI to earnings		7,000,000		4,000,000
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	(179,000,000)	345,000,000	849,000,000	(356,000,000)
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			366,000,000	(346,000,000)
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			483,000,000	(10,000,000)
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	2,000,000		(11,000,000)
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI		(16,000,000)		2,000,000
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	(1,000,000)		1,000,000
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI		25,000,000		36,000,000
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	36,000,000		108,000,000
Amount of (gain) loss reclassified from AOCI to earnings		38,000,000		132,000,000
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	6,000,000	178,000,000	4,000,000
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	(231,000,000)		(812,000,000)
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI		69,000,000		186,000,000
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		0
Amount of (gain) loss reclassified from AOCI to earnings		0		(1,000,000)
Commodity contracts \| Non-Qualifying Derivative Instruments
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	73,000,000	(113,000,000)	(26,000,000)	43,000,000
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			(15,000,000)	(16,000,000)
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			(418,000,000)	(42,000,000)
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			382,000,000	90,000,000
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			25,000,000	11,000,000
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	(1,000,000)		(22,000,000)
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI		4,000,000		1,000,000
Other contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of (gain) loss reclassified from AOCI to earnings	(1,000,000)		(4,000,000)
Amount of (gain) loss reclassified from AOCI to earnings		7,000,000		10,000,000
Other contracts \| Non-Qualifying Derivative Instruments
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain (loss) on derivatives	(1,000,000)	(8,000,000)	4,000,000	(10,000,000)
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	$ 1,000,000		$ 6,000,000
Cash flow hedge, amount of unrealized gain (loss) recognized in OCI		$ (10,000,000)		$ (12,000,000)