RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - EARNINGS AND COMPREHENSIVE INCOME (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
The Effect of Derivative Instruments on the Statements of Earnings and Comprehensive Income
Amount of unrealized gain/(loss) recognized in OCI	$ 20	$ 12	$ 141	$ 16
Amount of (gain)/loss reclassified from AOCI to earnings (effective portion)	35	(64)	83	(8)
Amount of (gain)/loss reclassified from AOCI to earnings (ineffective portion and amount excluded from effectiveness testing)	11	4	10	6
Estimated gain of AOCI related to cash flow hedges reclassified to earnings in the next 12 months			$ 11
Period to hedge exposures to the variability of cash flows for all forecasted transactions			30 months
Unrealized gain (loss) on derivative in fair value hedge	(4)	3	$ (12)	1
Unrealized gain (loss) on hedged item in fair value hedge	3	(3)	11	(1)
Total unrealized derivative fair value gain/(loss), net			(549)	898
Gain on derivative				1
Loss on derivative	1	2		2
Gain (loss) on derivative, net	0	0		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	(289)	480	(549)	898
Gain on derivative	2	1	2
Loss on derivative			1
Gain (loss) on derivative, net			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)	(2)	(102)	(3)	(101)
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	(277)	434	(701)	707
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			555	398
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			(146)	309
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	(3)	3	18	1
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	(5)	65	11	73
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)	43	36	84	84
Amount of (gain)/loss reclassified from AOCI to earnings (ineffective portion and amount excluded from effectiveness testing)	11	4	10	6
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	0	32	(2)	14
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	17	(41)	117	(55)
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)	0	(2)	(1)	(4)
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	(19)	19	156	182
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			3	(37)
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			10	197
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			127	9
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			16	13
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	(1)	(9)	(3)	12
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)	(6)	4	3	13
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	7	(5)	(2)	(5)
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	$ 12	$ (6)	$ (2)	$ (15)