RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Fair Value of Derivatives (Details) - CAD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Fair Value of Derivatives
Total net financial assets/(liabilities)	$ (576)	$ (1,069)
Foreign exchange contracts
Fair Value of Derivatives
Total net financial assets/(liabilities)	(287)	(299)
Interest rate contracts
Fair Value of Derivatives
Total net financial assets/(liabilities)	(5)	(610)
Commodity contracts
Fair Value of Derivatives
Total net financial assets/(liabilities)	(295)	(156)
Other contracts
Fair Value of Derivatives
Total net financial assets/(liabilities)	11	(4)
Recurring basis
Fair Value of Derivatives
Current derivative assets	553	323
Long-term derivative assets	502	561
Current derivative liabilities	(845)	(894)
Long-term derivative liabilities	(786)	(1,059)
Total net financial assets/(liabilities)	(576)	(1,069)
Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	225	180
Long-term derivative assets	248	466
Current derivative liabilities	(261)	(185)
Long-term derivative liabilities	(499)	(760)
Total net financial assets/(liabilities)	(287)	(299)
Recurring basis | Interest rate contracts
Fair Value of Derivatives
Long-term derivative assets	168	56
Current derivative liabilities	(35)	(425)
Long-term derivative liabilities	(138)	(241)
Total net financial assets/(liabilities)	(5)	(610)
Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	320	143
Long-term derivative assets	83	39
Current derivative liabilities	(549)	(280)
Long-term derivative liabilities	(149)	(58)
Total net financial assets/(liabilities)	(295)	(156)
Recurring basis | Other contracts
Fair Value of Derivatives
Current derivative assets	8
Long-term derivative assets	3
Current derivative liabilities		(4)
Total net financial assets/(liabilities)	11	(4)
Level 1 | Recurring basis
Fair Value of Derivatives
Current derivative assets	129	43
Long-term derivative assets	32	1
Current derivative liabilities	(149)	(39)
Long-term derivative liabilities	(35)	(1)
Total net financial assets/(liabilities)	(23)	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 financial assets/(liabilities)	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 financial assets/(liabilities)	0	0
Level 1 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	129	43
Long-term derivative assets	32	1
Current derivative liabilities	(149)	(39)
Long-term derivative liabilities	(35)	(1)
Total net financial assets/(liabilities)	(23)	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 financial assets/(liabilities)	0	0
Level 2 | Recurring basis
Fair Value of Derivatives
Current derivative assets	383	213
Long-term derivative assets	465	546
Current derivative liabilities	(496)	(632)
Long-term derivative liabilities	(671)	(1,009)
Total net financial assets/(liabilities)	(319)	(882)
Level 2 | Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	225	180
Long-term derivative assets	248	466
Current derivative liabilities	(261)	(185)
Long-term derivative liabilities	(499)	(760)
Total net financial assets/(liabilities)	(287)	(299)
Level 2 | Recurring basis | Interest rate contracts
Fair Value of Derivatives
Long-term derivative assets	168	56
Current derivative liabilities	(35)	(425)
Long-term derivative liabilities	(138)	(241)
Total net financial assets/(liabilities)	(5)	(610)
Level 2 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	150	33
Long-term derivative assets	46	24
Current derivative liabilities	(200)	(18)
Long-term derivative liabilities	(34)	(8)
Total net financial assets/(liabilities)	(38)	31
Level 2 | Recurring basis | Other contracts
Fair Value of Derivatives
Current derivative assets	8
Long-term derivative assets	3
Current derivative liabilities		(4)
Total net financial assets/(liabilities)	11	(4)
Level 3 | Recurring basis
Fair Value of Derivatives
Current derivative assets	41	67
Long-term derivative assets	5	14
Current derivative liabilities	(200)	(223)
Long-term derivative liabilities	(80)	(49)
Total net financial assets/(liabilities)	(234)	(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 financial assets/(liabilities)	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 financial assets/(liabilities)	0	0
Level 3 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	41	67
Long-term derivative assets	5	14
Current derivative liabilities	(200)	(223)
Long-term derivative liabilities	(80)	(49)
Total net financial assets/(liabilities)	(234)	(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 financial assets/(liabilities)	$ 0	$ 0