PENSION AND OTHER POSTRETIREMENT BENEFITS - AMOUNT RECOGNIZED IN ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - CAD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Canada \| Pension
Amount included in AOCI
Net actuarial loss	$ 435	$ 334
Prior service credit	0	0
Total amount recognized in AOCI	435	334
Canada \| OPEB
Amount included in AOCI
Net actuarial loss	(29)	17
Prior service credit	(2)	(2)
Total amount recognized in AOCI	(31)	15
United States \| Pension
Amount included in AOCI
Net actuarial loss	133	112
Prior service credit	(3)	0
Total amount recognized in AOCI	130	112
United States \| OPEB
Amount included in AOCI
Net actuarial loss	(15)	(15)
Prior service credit	(15)	(11)
Total amount recognized in AOCI	$ (30)	$ (26)

Amount, before tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (i-k)
-URI http://asc.fasb.org/extlink&oid=114867106&loc=SL7669646-108580

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-Section 45
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- Definition

Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
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- Definition

Amount, before tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 220
-SubTopic 10
-Section 45
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