RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Fair Value of Derivatives (Details) - CAD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Fair Value of Derivatives
Total net derivative asset/(liability)	$ (222)	$ (1,069)
Foreign exchange contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	93	(299)
Interest rate contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(145)	(610)
Commodity contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(180)	(156)
Other contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	10	(4)
Recurring basis
Fair Value of Derivatives
Current derivative assets	537	323
Long-term derivative assets	718	561
Current derivative liabilities	(798)	(894)
Long-term derivative liabilities	(679)	(1,059)
Total net derivative asset/(liability)	(222)	(1,069)
Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	312	180
Long-term derivative assets	493	466
Current derivative liabilities	(277)	(185)
Long-term derivative liabilities	(435)	(760)
Total net derivative asset/(liability)	93	(299)
Recurring basis | Interest rate contracts
Fair Value of Derivatives
Long-term derivative assets	157	56
Current derivative liabilities	(156)	(425)
Long-term derivative liabilities	(146)	(241)
Total net derivative asset/(liability)	(145)	(610)
Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	219	143
Long-term derivative assets	64	39
Current derivative liabilities	(365)	(280)
Long-term derivative liabilities	(98)	(58)
Total net derivative asset/(liability)	(180)	(156)
Recurring basis | Other contracts
Fair Value of Derivatives
Current derivative assets	6
Long-term derivative assets	4
Current derivative liabilities		(4)
Total net derivative asset/(liability)	10	(4)
Level 1 | Recurring basis
Fair Value of Derivatives
Current derivative assets	105	43
Long-term derivative assets	24	1
Current derivative liabilities	(123)	(39)
Long-term derivative liabilities	(28)	(1)
Total net derivative asset/(liability)	(22)	4
Level 1 | Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	0	0
Long-term derivative assets	0	0
Current derivative liabilities	0	0
Long-term derivative liabilities	0	0
Total net derivative asset/(liability)	0	0
Level 1 | Recurring basis | Interest rate contracts
Fair Value of Derivatives
Long-term derivative assets	0	0
Current derivative liabilities	0	0
Long-term derivative liabilities	0	0
Total net derivative asset/(liability)	0	0
Level 1 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	105	43
Long-term derivative assets	24	1
Current derivative liabilities	(123)	(39)
Long-term derivative liabilities	(28)	(1)
Total net derivative asset/(liability)	(22)	4
Level 1 | Recurring basis | Other contracts
Fair Value of Derivatives
Current derivative assets	0
Long-term derivative assets	0
Current derivative liabilities		0
Total net derivative asset/(liability)	0	0
Level 2 | Recurring basis
Fair Value of Derivatives
Current derivative assets	391	213
Long-term derivative assets	686	546
Current derivative liabilities	(502)	(632)
Long-term derivative liabilities	(597)	(1,009)
Total net derivative asset/(liability)	(22)	(882)
Level 2 | Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	312	180
Long-term derivative assets	493	466
Current derivative liabilities	(277)	(185)
Long-term derivative liabilities	(435)	(760)
Total net derivative asset/(liability)	93	(299)
Level 2 | Recurring basis | Interest rate contracts
Fair Value of Derivatives
Long-term derivative assets	157	56
Current derivative liabilities	(156)	(425)
Long-term derivative liabilities	(146)	(241)
Total net derivative asset/(liability)	(145)	(610)
Level 2 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	73	33
Long-term derivative assets	32	24
Current derivative liabilities	(69)	(18)
Long-term derivative liabilities	(16)	(8)
Total net derivative asset/(liability)	20	31
Level 2 | Recurring basis | Other contracts
Fair Value of Derivatives
Current derivative assets	6
Long-term derivative assets	4
Current derivative liabilities		(4)
Total net derivative asset/(liability)	10	(4)
Level 3 | Recurring basis
Fair Value of Derivatives
Current derivative assets	41	67
Long-term derivative assets	8	14
Current derivative liabilities	(173)	(223)
Long-term derivative liabilities	(54)	(49)
Total net derivative asset/(liability)	(178)	(191)
Level 3 | Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	0	0
Long-term derivative assets	0	0
Current derivative liabilities	0	0
Long-term derivative liabilities	0	0
Total net derivative asset/(liability)	0	0
Level 3 | Recurring basis | Interest rate contracts
Fair Value of Derivatives
Long-term derivative assets	0	0
Current derivative liabilities	0	0
Long-term derivative liabilities	0	0
Total net derivative asset/(liability)	0	0
Level 3 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	41	67
Long-term derivative assets	8	14
Current derivative liabilities	(173)	(223)
Long-term derivative liabilities	(54)	(49)
Total net derivative asset/(liability)	(178)	(191)
Level 3 | Recurring basis | Other contracts
Fair Value of Derivatives
Current derivative assets	0
Long-term derivative assets	0
Current derivative liabilities		0
Total net derivative asset/(liability)	$ 0	$ 0