RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - FAIR VALUE OF DERIVATIVES (Details) - CAD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair Value of Derivatives
Total net derivative asset/(liability)	$ (2,607)	$ (2,192)
Foreign exchange contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(2,058)	(1,383)
Interest rate contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(239)	(348)
Commodity contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(305)	(457)
Other contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(5)	(4)
Recurring basis
Fair Value of Derivatives
Current derivative assets	223	296
Long-term derivative assets	103	181
Current derivative liabilities	(996)	(1,130)
Long-term derivative liabilities	(1,937)	(1,539)
Total net derivative asset/(liability)	(2,607)	(2,192)
Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	74	143
Long-term derivative assets	52	145
Current derivative liabilities	(426)	(359)
Long-term derivative liabilities	(1,758)	(1,312)
Total net derivative asset/(liability)	(2,058)	(1,383)
Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative assets	37	8
Long-term derivative assets	19	13
Current derivative liabilities	(276)	(329)
Long-term derivative liabilities	(19)	(40)
Total net derivative asset/(liability)	(239)	(348)
Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	112	145
Long-term derivative assets	31	23
Current derivative liabilities	(290)	(439)
Long-term derivative liabilities	(158)	(186)
Total net derivative asset/(liability)	(305)	(457)
Recurring basis | Other contracts
Fair Value of Derivatives
Long-term derivative assets	1
Current derivative liabilities	(4)	(3)
Long-term derivative liabilities	(2)	(1)
Total net derivative asset/(liability)	(5)	(4)
Level 1 | Recurring basis
Fair Value of Derivatives
Current derivative assets	1	1
Current derivative liabilities	(20)	(13)
Total net derivative asset/(liability)	(19)	(12)
Level 1 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	1	1
Current derivative liabilities	(20)	(13)
Total net derivative asset/(liability)	(19)	(12)
Level 2 | Recurring basis
Fair Value of Derivatives
Current derivative assets	119	181
Long-term derivative assets	76	160
Current derivative liabilities	(757)	(778)
Long-term derivative liabilities	(1,792)	(1,356)
Total net derivative asset/(liability)	(2,354)	(1,793)
Level 2 | Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	74	143
Long-term derivative assets	52	145
Current derivative liabilities	(426)	(359)
Long-term derivative liabilities	(1,758)	(1,312)
Total net derivative asset/(liability)	(2,058)	(1,383)
Level 2 | Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative assets	37	8
Long-term derivative assets	19	13
Current derivative liabilities	(276)	(329)
Long-term derivative liabilities	(19)	(40)
Total net derivative asset/(liability)	(239)	(348)
Level 2 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	8	30
Long-term derivative assets	4	2
Current derivative liabilities	(51)	(87)
Long-term derivative liabilities	(13)	(3)
Total net derivative asset/(liability)	(52)	(58)
Level 2 | Recurring basis | Other contracts
Fair Value of Derivatives
Long-term derivative assets	1
Current derivative liabilities	(4)	(3)
Long-term derivative liabilities	(2)	(1)
Total net derivative asset/(liability)	(5)	(4)
Level 3 | Recurring basis
Fair Value of Derivatives
Current derivative assets	103	114
Long-term derivative assets	27	21
Current derivative liabilities	(219)	(339)
Long-term derivative liabilities	(145)	(183)
Total net derivative asset/(liability)	(234)	(387)
Level 3 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	103	114
Long-term derivative assets	27	21
Current derivative liabilities	(219)	(339)
Long-term derivative liabilities	(145)	(183)
Total net derivative asset/(liability)	$ (234)	$ (387)