RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - TOTAL DERIVATIVE INSTRUMENTS (Details) - CAD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	$ (2,607)	$ (2,192)
Derivative liabilities, Total Net Derivative Instruments	(2,607)	(2,192)
Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(2,540)	(1,991)
Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(19)	(155)
Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(35)	(46)
Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(13)
Accounts receivable and other
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	223	296
Derivative assets, Amounts Available for Offset	(127)	(150)
Derivative assets, Total Net Derivative Instruments	96	146
Accounts receivable and other | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	184	281
Accounts receivable and other | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	37	9
Accounts receivable and other | Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	2	4
Accounts receivable and other | Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented		2
Deferred amounts and other assets
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	103	181
Derivative assets, Amounts Available for Offset	(64)	(146)
Derivative assets, Total Net Derivative Instruments	39	35
Deferred amounts and other assets | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	54	149
Deferred amounts and other assets | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	49	25
Deferred amounts and other assets | Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented		1
Deferred amounts and other assets | Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented		6
Accounts payable and other
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(996)	(1,130)
Derivative liabilities, Amounts Available for Offset	127	150
Derivative liabilities, Total Net Derivative Instruments	(869)	(980)
Accounts payable and other | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(873)	(936)
Accounts payable and other | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(94)	(146)
Accounts payable and other | Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(25)	(42)
Accounts payable and other | Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(4)	(6)
Other long-term liabilities
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(1,937)	(1,539)
Derivative liabilities, Amounts Available for Offset	64	146
Derivative liabilities, Total Net Derivative Instruments	(1,873)	(1,393)
Other long-term liabilities | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(1,905)	(1,485)
Other long-term liabilities | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(11)	(43)
Other long-term liabilities | Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(12)	(9)
Other long-term liabilities | Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(9)	(2)
Foreign exchange contracts
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(2,058)	(1,383)
Derivative liabilities, Total Net Derivative Instruments	(2,058)	(1,383)
Foreign exchange contracts | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(2,031)	(1,330)
Foreign exchange contracts | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	8	(7)
Foreign exchange contracts | Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(35)	(46)
Foreign exchange contracts | Accounts receivable and other
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	74	143
Derivative assets, Amounts Available for Offset	(48)	(83)
Derivative assets, Total Net Derivative Instruments	26	60
Foreign exchange contracts | Accounts receivable and other | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	72	138
Foreign exchange contracts | Accounts receivable and other | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented		1
Foreign exchange contracts | Accounts receivable and other | Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	2	4
Foreign exchange contracts | Deferred amounts and other assets
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	52	145
Derivative assets, Amounts Available for Offset	(34)	(125)
Derivative assets, Total Net Derivative Instruments	18	20
Foreign exchange contracts | Deferred amounts and other assets | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	39	143
Foreign exchange contracts | Deferred amounts and other assets | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	13	1
Foreign exchange contracts | Deferred amounts and other assets | Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented		1
Foreign exchange contracts | Accounts payable and other
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(426)	(359)
Derivative liabilities, Amounts Available for Offset	48	83
Derivative liabilities, Total Net Derivative Instruments	(378)	(276)
Foreign exchange contracts | Accounts payable and other | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(396)	(312)
Foreign exchange contracts | Accounts payable and other | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(5)	(5)
Foreign exchange contracts | Accounts payable and other | Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(25)	(42)
Foreign exchange contracts | Other long-term liabilities
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(1,758)	(1,312)
Derivative liabilities, Amounts Available for Offset	34	125
Derivative liabilities, Total Net Derivative Instruments	(1,724)	(1,187)
Foreign exchange contracts | Other long-term liabilities | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(1,746)	(1,299)
Foreign exchange contracts | Other long-term liabilities | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented		(4)
Foreign exchange contracts | Other long-term liabilities | Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(12)	(9)
Interest rate contracts
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(239)	(348)
Derivative liabilities, Total Net Derivative Instruments	(239)	(348)
Interest rate contracts | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(185)	(183)
Interest rate contracts | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(41)	(165)
Interest rate contracts | Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(13)
Interest rate contracts | Accounts receivable and other
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	37	8
Derivative assets, Amounts Available for Offset	(5)	(3)
Derivative assets, Total Net Derivative Instruments	32	5
Interest rate contracts | Accounts receivable and other | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	37	6
Interest rate contracts | Accounts receivable and other | Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented		2
Interest rate contracts | Deferred amounts and other assets
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	19	13
Derivative assets, Amounts Available for Offset		(2)
Derivative assets, Total Net Derivative Instruments	19	11
Interest rate contracts | Deferred amounts and other assets | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	19	7
Interest rate contracts | Deferred amounts and other assets | Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented		6
Interest rate contracts | Accounts payable and other
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(276)	(329)
Derivative liabilities, Amounts Available for Offset	5	3
Derivative liabilities, Total Net Derivative Instruments	(271)	(326)
Interest rate contracts | Accounts payable and other | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(185)	(183)
Interest rate contracts | Accounts payable and other | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(87)	(140)
Interest rate contracts | Accounts payable and other | Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(4)	(6)
Interest rate contracts | Other long-term liabilities
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(19)	(40)
Derivative liabilities, Amounts Available for Offset		2
Derivative liabilities, Total Net Derivative Instruments	(19)	(38)
Interest rate contracts | Other long-term liabilities | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented		0
Interest rate contracts | Other long-term liabilities | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(10)	(38)
Interest rate contracts | Other long-term liabilities | Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(9)	(2)
Commodity contracts
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(305)	(457)
Derivative liabilities, Total Net Derivative Instruments	(305)	(457)
Commodity contracts | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(320)	(476)
Commodity contracts | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	15	19
Commodity contracts | Accounts receivable and other
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	112	145
Derivative assets, Amounts Available for Offset	(74)	(64)
Derivative assets, Total Net Derivative Instruments	38	81
Commodity contracts | Accounts receivable and other | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	112	143
Commodity contracts | Accounts receivable and other | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	0	2
Commodity contracts | Deferred amounts and other assets
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	31	23
Derivative assets, Amounts Available for Offset	(29)	(19)
Derivative assets, Total Net Derivative Instruments	2	4
Commodity contracts | Deferred amounts and other assets | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	15	6
Commodity contracts | Deferred amounts and other assets | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	16	17
Commodity contracts | Accounts payable and other
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(290)	(439)
Derivative liabilities, Amounts Available for Offset	74	64
Derivative liabilities, Total Net Derivative Instruments	(216)	(375)
Commodity contracts | Accounts payable and other | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(289)	(439)
Commodity contracts | Accounts payable and other | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(1)
Commodity contracts | Other long-term liabilities
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(158)	(186)
Derivative liabilities, Amounts Available for Offset	29	19
Derivative liabilities, Total Net Derivative Instruments	(129)	(167)
Commodity contracts | Other long-term liabilities | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(158)	(186)
Other contracts
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(5)	(4)
Derivative liabilities, Total Net Derivative Instruments	(5)	(4)
Other contracts | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(4)	(2)
Other contracts | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(1)	(2)
Other contracts | Deferred amounts and other assets
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	1
Derivative assets, Amounts Available for Offset	(1)
Derivative assets, Total Net Derivative Instruments	0
Other contracts | Deferred amounts and other assets | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	1
Other contracts | Accounts payable and other
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(4)	(3)
Derivative liabilities, Total Net Derivative Instruments	(4)	(3)
Other contracts | Accounts payable and other | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(3)	(2)
Other contracts | Accounts payable and other | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(1)	(1)
Other contracts | Other long-term liabilities
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(2)	(1)
Derivative liabilities, Amounts Available for Offset	1
Derivative liabilities, Total Net Derivative Instruments	(1)	(1)
Other contracts | Other long-term liabilities | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(1)
Other contracts | Other long-term liabilities | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	$ (1)	$ (1)