RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - FAIR VALUE OF DERIVATIVES (Details) - CAD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value of Derivatives
Short-term portion of derivative assets	$ 327	$ 498
Current derivative liabilities	(920)	(1,234)
Total net derivative asset/(liability)	(1,687)	(3,210)
Forward currency contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(1,102)	(2,718)
Interest rate contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(534)	(515)
Commodity contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(58)	30
Other contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	7	(7)
Fair Value
Fair Value of Derivatives
Short-term portion of derivative assets	328	498
Long-term derivative assets	101	119
Current derivative liabilities	(936)	(1,234)
Long-term derivative liabilities	(1,180)	(2,593)
Total net derivative asset/(liability)	(1,687)	(3,210)
Fair Value | Forward currency contracts
Fair Value of Derivatives
Short-term portion of derivative assets	161	47
Long-term derivative assets	81	62
Current derivative liabilities	(410)	(615)
Long-term derivative liabilities	(934)	(2,212)
Total net derivative asset/(liability)	(1,102)	(2,718)
Fair Value | Interest rate contracts
Fair Value of Derivatives
Short-term portion of derivative assets		22
Long-term derivative assets		5
Current derivative liabilities	(353)	(341)
Long-term derivative liabilities	(181)	(201)
Total net derivative asset/(liability)	(534)	(515)
Fair Value | Commodity contracts
Fair Value of Derivatives
Short-term portion of derivative assets	163	429
Long-term derivative assets	17	52
Current derivative liabilities	(173)	(273)
Long-term derivative liabilities	(65)	(178)
Total net derivative asset/(liability)	(58)	30
Fair Value | Other contracts
Fair Value of Derivatives
Short-term portion of derivative assets	4
Long-term derivative assets	3
Current derivative liabilities	0	(5)
Long-term derivative liabilities	0	(2)
Total net derivative asset/(liability)		(7)
Level 1 | Fair Value
Fair Value of Derivatives
Short-term portion of derivative assets	0	24
Long-term derivative assets	0	0
Current derivative liabilities	(5)	(7)
Long-term derivative liabilities	0	0
Total net derivative asset/(liability)	(5)	17
Level 1 | Fair Value | Forward currency contracts
Fair Value of Derivatives
Short-term portion of 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 | Fair Value | Interest rate contracts
Fair Value of Derivatives
Short-term portion of 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 | Fair Value | Commodity contracts
Fair Value of Derivatives
Short-term portion of derivative assets	0	24
Long-term derivative assets	0	0
Current derivative liabilities	(5)	(7)
Long-term derivative liabilities	0	0
Total net derivative asset/(liability)	(5)	17
Level 1 | Fair Value | Other contracts
Fair Value of Derivatives
Short-term portion of 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 2 | Fair Value
Fair Value of Derivatives
Short-term portion of derivative assets	198	114
Long-term derivative assets	96	97
Current derivative liabilities	(786)	(989)
Long-term derivative liabilities	(1,121)	(2,438)
Total net derivative asset/(liability)	(1,613)	(3,216)
Level 2 | Fair Value | Forward currency contracts
Fair Value of Derivatives
Short-term portion of derivative assets	161	47
Long-term derivative assets	81	62
Current derivative liabilities	(410)	(615)
Long-term derivative liabilities	(934)	(2,212)
Total net derivative asset/(liability)	(1,102)	(2,718)
Level 2 | Fair Value | Interest rate contracts
Fair Value of Derivatives
Short-term portion of derivative assets		22
Long-term derivative assets		5
Current derivative liabilities	(353)	(341)
Long-term derivative liabilities	(181)	(201)
Total net derivative asset/(liability)	(534)	(515)
Level 2 | Fair Value | Commodity contracts
Fair Value of Derivatives
Short-term portion of derivative assets	33	45
Long-term derivative assets	12	30
Current derivative liabilities	(23)	(28)
Long-term derivative liabilities	(6)	(23)
Total net derivative asset/(liability)	16	24
Level 2 | Fair Value | Other contracts
Fair Value of Derivatives
Short-term portion of derivative assets	4
Long-term derivative assets	3
Current derivative liabilities	0	(5)
Long-term derivative liabilities	0	(2)
Total net derivative asset/(liability)	7	(7)
Level 3 | Fair Value
Fair Value of Derivatives
Short-term portion of derivative assets	130	360
Long-term derivative assets	5	22
Current derivative liabilities	(145)	(238)
Long-term derivative liabilities	(59)	(155)
Total net derivative asset/(liability)	(69)	(11)
Level 3 | Fair Value | Forward currency contracts
Fair Value of Derivatives
Short-term portion of 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 | Fair Value | Interest rate contracts
Fair Value of Derivatives
Short-term portion of 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 3 | Fair Value | Commodity contracts
Fair Value of Derivatives
Short-term portion of derivative assets	130	360
Long-term derivative assets	5	22
Current derivative liabilities	(145)	(238)
Long-term derivative liabilities	(59)	(155)
Total net derivative asset/(liability)	(69)	(11)
Level 3 | Fair Value | Other contracts
Fair Value of Derivatives
Short-term portion of 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