RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Fair Value of Derivatives (Details) - CAD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Fair Value of Derivatives
Total net derivative asset/(liability)	$ (3,664)	$ (1,687)
Foreign exchange contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(2,874)	(1,102)
Interest rate contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(1,226)	(534)
Commodity contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	444	(58)
Other contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(8)	7
Recurring basis
Fair Value of Derivatives
Current derivative assets	1,068	328
Long-term derivative assets	441	101
Current derivative liabilities	(2,262)	(936)
Long-term derivative liabilities	(2,911)	(1,180)
Total net derivative asset/(liability)	(3,664)	(1,687)
Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	105	161
Long-term derivative assets	373	81
Current derivative liabilities	(924)	(410)
Long-term derivative liabilities	(2,428)	(934)
Total net derivative asset/(liability)	(2,874)	(1,102)
Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative liabilities	(831)	(353)
Long-term derivative liabilities	(395)	(181)
Total net derivative asset/(liability)	(1,226)	(534)
Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	963	163
Long-term derivative assets	68	17
Current derivative liabilities	(506)	(173)
Long-term derivative liabilities	(81)	(65)
Total net derivative asset/(liability)	444	(58)
Recurring basis | Other contracts
Fair Value of Derivatives
Current derivative assets		4
Long-term derivative assets		3
Current derivative liabilities	(1)
Long-term derivative liabilities	(7)
Total net derivative asset/(liability)	(8)	7
Level 1 | Recurring basis
Fair Value of Derivatives
Current derivative assets	90	0
Long-term derivative assets	33	0
Current derivative liabilities	(58)	(5)
Long-term derivative liabilities	(13)	0
Total net derivative asset/(liability)	52	(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 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	90	0
Long-term derivative assets	33	0
Current derivative liabilities	(58)	(5)
Long-term derivative liabilities	(13)	0
Total net derivative asset/(liability)	52	(5)
Level 1 | Recurring basis | Other contracts
Fair Value of Derivatives
Current derivative assets		0
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	178	198
Long-term derivative assets	392	96
Current derivative liabilities	(1,776)	(786)
Long-term derivative liabilities	(2,838)	(1,121)
Total net derivative asset/(liability)	(4,044)	(1,613)
Level 2 | Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	105	161
Long-term derivative assets	373	81
Current derivative liabilities	(924)	(410)
Long-term derivative liabilities	(2,428)	(934)
Total net derivative asset/(liability)	(2,874)	(1,102)
Level 2 | Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative liabilities	(831)	(353)
Long-term derivative liabilities	(395)	(181)
Total net derivative asset/(liability)	(1,226)	(534)
Level 2 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	73	33
Long-term derivative assets	19	12
Current derivative liabilities	(20)	(23)
Long-term derivative liabilities	(8)	(6)
Total net derivative asset/(liability)	64	16
Level 2 | Recurring basis | Other contracts
Fair Value of Derivatives
Current derivative assets		4
Long-term derivative assets		3
Current derivative liabilities	(1)
Long-term derivative liabilities	(7)
Total net derivative asset/(liability)	(8)	7
Level 3 | Recurring basis
Fair Value of Derivatives
Current derivative assets	800	130
Long-term derivative assets	16	5
Current derivative liabilities	(428)	(145)
Long-term derivative liabilities	(60)	(59)
Total net derivative asset/(liability)	328	(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 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	800	130
Long-term derivative assets	16	5
Current derivative liabilities	(428)	(145)
Long-term derivative liabilities	(60)	(59)
Total net derivative asset/(liability)	328	(69)
Level 3 | Recurring basis | Other contracts
Fair Value of Derivatives
Current derivative assets		0
Long-term derivative assets		0
Current derivative liabilities	0
Long-term derivative liabilities	0
Total net derivative asset/(liability)	$ 0	$ 0