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RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - EFFECTS ON EARNINGS AND COMPREHENSIVE INCOME (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of unrealized gain/(loss) recognized in OCI	$ (591)
Amount of unrealized gains/(loss) recognized in OCI		$ (141)	$ 297
Amount (gain)/loss reclassified from AOCI to earnings	158
Amount of (gains)/loss reclassified from AOCI to earnings (effective portion)		191	279
Unrealized gain on derivative	1,751	(903)	1,242
Unrealized gain on derivative		7
Unrealized gain on hedged item		1
Realized loss on derivative		(8)
Realized loss on hedged item		(1)
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	$ 80
Period to hedge exposures to the variability of cash flows for all forecasted transactions	24 months
Forward currency contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount (gain)/loss reclassified from AOCI to earnings	$ 5
Forward currency contracts \| Other income/(expense)
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of (gains)/loss reclassified from AOCI to earnings (effective portion)		5	(104)
Forward currency 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	(19)
Amount of unrealized gains/(loss) recognized in OCI		19	(5)
Forward currency 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	2
Amount of unrealized gains/(loss) recognized in OCI		31	284
Interest rate contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount (gain)/loss reclassified from AOCI to earnings	157
Interest rate contracts \| Interest expense
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of (gains)/loss reclassified from AOCI to earnings (effective portion)		184	384
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	(559)
Amount of unrealized gains/(loss) recognized in OCI		(190)	6
Commodity contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount (gain)/loss reclassified from AOCI to earnings	(1)
Commodity contracts \| Commodity costs
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of (gains)/loss reclassified from AOCI to earnings (effective portion)		(1)	(9)
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	(25)
Amount of unrealized gains/(loss) recognized in OCI		2	11
Other contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount (gain)/loss reclassified from AOCI to earnings	(3)
Other contracts \| Operating and administrative expense
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of (gains)/loss reclassified from AOCI to earnings (effective portion)		3	8
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	10
Amount of unrealized gains/(loss) recognized in OCI		(3)	1
Interest Rate Swap \| Interest expense
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of (gains)/loss reclassified from AOCI to earnings (effective portion)			296
Non-Qualifying Derivative Instruments \| Forward currency contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain on derivative	1,626	(1,390)	1,284
Non-Qualifying Derivative Instruments \| Forward currency contracts \| Transportation and other services revenues
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain on derivative	930	(1,108)	800
Non-Qualifying Derivative Instruments \| Forward currency contracts \| Other income/(expense)
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain on derivative	696	(282)	484
Non-Qualifying Derivative Instruments \| Interest rate contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain on derivative	178	5	157
Non-Qualifying Derivative Instruments \| Commodity contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain on derivative	(62)	485	(199)
Non-Qualifying Derivative Instruments \| Commodity contracts \| Transportation and other services revenues
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain on derivative	(26)	66	(104)
Non-Qualifying Derivative Instruments \| Commodity contracts \| Commodity sales
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain on derivative	(544)	599	90
Non-Qualifying Derivative Instruments \| Commodity contracts \| Commodity costs
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain on derivative	459	(193)	(223)
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 on derivative	49	13	38
Non-Qualifying Derivative Instruments \| Other contracts
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Unrealized gain on derivative	$ 9	$ (3)	$ 0