RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Derivative Assets and Liabilities Measured on a Recurring Basis (Details) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Fair Value Hierarchy
Derivative Instrument Assets:	CAD 509	CAD 610
Derivative Instrument Liabilities:	(937)	(1,551)
Commodity contract \| Power
Fair Value Hierarchy
Derivative Instrument Assets:	479	509
Derivative Instrument Liabilities:	(448)	(717)
Foreign exchange contracts
Fair Value Hierarchy
Derivative Instrument Assets:	26	96
Derivative Instrument Liabilities:	(486)	(828)
Interest rate
Fair Value Hierarchy
Derivative Instrument Assets:	4	5
Derivative Instrument Liabilities:	(3)	(6)
Recurring basis
Fair Value Hierarchy
Assets and (liabilities), net	(428)	(941)
Recurring basis \| Commodity contract \| Power
Fair Value Hierarchy
Derivative Instrument Assets:	479	509
Derivative Instrument Liabilities:	(448)	(717)
Recurring basis \| Foreign exchange contracts
Fair Value Hierarchy
Derivative Instrument Assets:	26	96
Derivative Instrument Liabilities:	(486)	(828)
Recurring basis \| Interest rate
Fair Value Hierarchy
Derivative Instrument Assets:	4	5
Derivative Instrument Liabilities:	(3)	(6)
Recurring basis \| Quoted Prices in Active Markets (Level I)
Fair Value Hierarchy
Assets and (liabilities), net	32	(68)
Recurring basis \| Quoted Prices in Active Markets (Level I) \| Commodity contract \| Power
Fair Value Hierarchy
Derivative Instrument Assets:	134	34
Derivative Instrument Liabilities:	(102)	(102)
Recurring basis \| Significant Other Observable Inputs (Level II)
Fair Value Hierarchy
Assets and (liabilities), net	(476)	(882)
Recurring basis \| Significant Other Observable Inputs (Level II) \| Commodity contract \| Power
Fair Value Hierarchy
Derivative Instrument Assets:	326	462
Derivative Instrument Liabilities:	(343)	(611)
Recurring basis \| Significant Other Observable Inputs (Level II) \| Foreign exchange contracts
Fair Value Hierarchy
Derivative Instrument Assets:	26	96
Derivative Instrument Liabilities:	(486)	(828)
Recurring basis \| Significant Other Observable Inputs (Level II) \| Interest rate
Fair Value Hierarchy
Derivative Instrument Assets:	4	5
Derivative Instrument Liabilities:	(3)	(6)
Recurring basis \| Significant Unobservable Inputs (Level III)
Fair Value Hierarchy
Assets and (liabilities), net	16	9
Recurring basis \| Significant Unobservable Inputs (Level III) \| Commodity contract \| Power
Fair Value Hierarchy
Derivative Instrument Assets:	19	13
Derivative Instrument Liabilities:	CAD (3)	CAD (4)

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
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-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624163-113959

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226000-175313

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
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-Section 50
-Paragraph 4D
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-URI http://asc.fasb.org/extlink&oid=64830801&loc=SL51823488-111719

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=64830801&loc=SL51823488-111719

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-Topic 815
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-Paragraph 4B
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-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624163-113959

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226000-175313

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