RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Fair Value of Derivatives (Details) - CAD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fair Value of Derivatives
Total net derivative asset/(liability)	$ (2,020)	$ (1,687)
Foreign exchange contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(1,241)	(1,102)
Interest rate contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(753)	(534)
Commodity contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(15)	(58)
Other contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(11)	7
Recurring basis
Fair Value of Derivatives
Current derivative assets	215	328
Long-term derivative assets	300	101
Current derivative liabilities	(717)	(936)
Long-term derivative liabilities	(1,818)	(1,180)
Total net derivative asset/(liability)	(2,020)	(1,687)
Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	56	161
Long-term derivative assets	226	81
Current derivative liabilities	(383)	(410)
Long-term derivative liabilities	(1,140)	(934)
Total net derivative asset/(liability)	(1,241)	(1,102)
Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative assets		163
Long-term derivative assets	8
Current derivative liabilities	(172)	(353)
Long-term derivative liabilities	(589)	(181)
Total net derivative asset/(liability)	(753)	(534)
Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	159	4
Long-term derivative assets	66	17
Current derivative liabilities	(160)	(173)
Long-term derivative liabilities	(80)	(65)
Total net derivative asset/(liability)	(15)	(58)
Recurring basis | Other contracts
Fair Value of Derivatives
Long-term derivative assets		3
Current derivative liabilities	(2)
Long-term derivative liabilities	(9)
Total net derivative asset/(liability)	(11)	7
Level 1 | Recurring basis
Fair Value of Derivatives
Current derivative assets	16	0
Long-term derivative assets	13	0
Current derivative liabilities	(16)	(5)
Long-term derivative liabilities	(11)	0
Total net derivative asset/(liability)	2	(5)
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
Current derivative assets		0
Long-term derivative assets	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	16	0
Long-term derivative assets	13	0
Current derivative liabilities	(16)	(5)
Long-term derivative liabilities	(11)	0
Total net derivative asset/(liability)	2	(5)
Level 1 | Recurring basis | Other contracts
Fair Value of Derivatives
Long-term derivative assets		0
Current derivative liabilities	0
Long-term derivative liabilities	0
Total net derivative asset/(liability)	0	0
Level 2 | Recurring basis
Fair Value of Derivatives
Current derivative assets	103	198
Long-term derivative assets	281	96
Current derivative liabilities	(585)	(786)
Long-term derivative liabilities	(1,757)	(1,121)
Total net derivative asset/(liability)	(1,958)	(1,613)
Level 2 | Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	56	161
Long-term derivative assets	226	81
Current derivative liabilities	(383)	(410)
Long-term derivative liabilities	(1,140)	(934)
Total net derivative asset/(liability)	(1,241)	(1,102)
Level 2 | Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative assets		33
Long-term derivative assets	8
Current derivative liabilities	(172)	(353)
Long-term derivative liabilities	(589)	(181)
Total net derivative asset/(liability)	(753)	(534)
Level 2 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	47	4
Long-term derivative assets	47	12
Current derivative liabilities	(28)	(23)
Long-term derivative liabilities	(19)	(6)
Total net derivative asset/(liability)	47	16
Level 2 | Recurring basis | Other contracts
Fair Value of Derivatives
Long-term derivative assets		3
Current derivative liabilities	(2)
Long-term derivative liabilities	(9)
Total net derivative asset/(liability)	(11)	7
Level 3 | Recurring basis
Fair Value of Derivatives
Current derivative assets	96	130
Long-term derivative assets	6	5
Current derivative liabilities	(116)	(145)
Long-term derivative liabilities	(50)	(59)
Total net derivative asset/(liability)	(64)	(69)
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
Current derivative assets		130
Long-term derivative assets	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	96	0
Long-term derivative assets	6	5
Current derivative liabilities	(116)	(145)
Long-term derivative liabilities	(50)	(59)
Total net derivative asset/(liability)	(64)	(69)
Level 3 | Recurring basis | Other contracts
Fair Value of Derivatives
Long-term derivative assets		0
Current derivative liabilities	0
Long-term derivative liabilities	0
Total net derivative asset/(liability)	$ 0	$ 0