RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Fair Value of Derivatives (Details) - CAD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Fair Value of Derivatives
Total net derivative asset/(liability)	$ (2,835)	$ (3,210)
Foreign exchange contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(1,873)	(2,718)
Interest rate contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(943)	(515)
Commodity contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(18)	30
Other contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(1)	(7)
Recurring basis
Fair Value of Derivatives
Current derivative assets	262	498
Long-term derivative assets	177	119
Current derivative liabilities	(952)	(1,234)
Long-term derivative liabilities	(2,322)	(2,593)
Total net derivative asset/(liability)	(2,835)	(3,210)
Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	76	47
Long-term derivative assets	140	62
Current derivative liabilities	(553)	(615)
Long-term derivative liabilities	(1,536)	(2,212)
Total net derivative asset/(liability)	(1,873)	(2,718)
Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative assets	1	22
Long-term derivative assets		5
Current derivative liabilities	(258)	(341)
Long-term derivative liabilities	(686)	(201)
Total net derivative asset/(liability)	(943)	(515)
Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	185	429
Long-term derivative assets	35	52
Current derivative liabilities	(140)	(273)
Long-term derivative liabilities	(98)	(178)
Total net derivative asset/(liability)	(18)	30
Recurring basis | Other contracts
Fair Value of Derivatives
Long-term derivative assets	2
Current derivative liabilities	(1)	(5)
Long-term derivative liabilities	(2)	(2)
Total net derivative asset/(liability)	(1)	(7)
Level 1 | Recurring basis
Fair Value of Derivatives
Current derivative assets	9	24
Long-term derivative assets	0	0
Current derivative liabilities	(2)	(7)
Long-term derivative liabilities	0	0
Total net derivative asset/(liability)	7	17
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	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	9	24
Long-term derivative assets	0	0
Current derivative liabilities	(2)	(7)
Long-term derivative liabilities	0	0
Total net derivative asset/(liability)	7	17
Level 1 | Recurring basis | Other contracts
Fair Value of Derivatives
Long-term derivative assets	0
Current derivative liabilities	0	0
Long-term derivative liabilities	0	0
Total net derivative asset/(liability)	0	0
Level 2 | Recurring basis
Fair Value of Derivatives
Current derivative assets	121	114
Long-term derivative assets	165	97
Current derivative liabilities	(833)	(989)
Long-term derivative liabilities	(2,231)	(2,438)
Total net derivative asset/(liability)	(2,778)	(3,216)
Level 2 | Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	76	47
Long-term derivative assets	140	62
Current derivative liabilities	(553)	(615)
Long-term derivative liabilities	(1,536)	(2,212)
Total net derivative asset/(liability)	(1,873)	(2,718)
Level 2 | Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative assets	1	22
Long-term derivative assets		5
Current derivative liabilities	(258)	(341)
Long-term derivative liabilities	(686)	(201)
Total net derivative asset/(liability)	(943)	(515)
Level 2 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	44	45
Long-term derivative assets	23	30
Current derivative liabilities	(21)	(28)
Long-term derivative liabilities	(7)	(23)
Total net derivative asset/(liability)	39	24
Level 2 | Recurring basis | Other contracts
Fair Value of Derivatives
Long-term derivative assets	2
Current derivative liabilities	(1)	(5)
Long-term derivative liabilities	(2)	(2)
Total net derivative asset/(liability)	(1)	(7)
Level 3 | Recurring basis
Fair Value of Derivatives
Current derivative assets	132	360
Long-term derivative assets	12	22
Current derivative liabilities	(117)	(238)
Long-term derivative liabilities	(91)	(155)
Total net derivative asset/(liability)	(64)	(11)
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	0	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 3 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	132	360
Long-term derivative assets	12	22
Current derivative liabilities	(117)	(238)
Long-term derivative liabilities	(91)	(155)
Total net derivative asset/(liability)	(64)	(11)
Level 3 | Recurring basis | Other contracts
Fair Value of Derivatives
Long-term derivative assets	0
Current derivative liabilities	0	0
Long-term derivative liabilities	0	0
Total net derivative asset/(liability)	$ 0	$ 0