PENSION AND OTHER POSTRETIREMENT BENEFITS - EXPECTED BENEFIT PAYMENTS AND EMPLOYER CONTRIBUTIONS (Details) CAD in Millions	Dec. 31, 2017 CAD
Canada \| Pension
Benefits Expected to be Paid by the Company
Expected future benefit payments for 2018	CAD 158
Expected future benefit payments for 2019	165
Expected future benefit payments for 2020	172
Expected future benefit payments for 2021	180
Expected future benefit payments for 2022	187
Expected future benefit payments for 2023-2027	1,036
Contributions expected to be paid in next fiscal year	36
Canada \| OPEB
Benefits Expected to be Paid by the Company
Expected future benefit payments for 2018	12
Expected future benefit payments for 2019	12
Expected future benefit payments for 2020	13
Expected future benefit payments for 2021	13
Expected future benefit payments for 2022	14
Expected future benefit payments for 2023-2027	43
Contributions expected to be paid in next fiscal year	7
United States \| Pension
Benefits Expected to be Paid by the Company
Expected future benefit payments for 2018	82
Expected future benefit payments for 2019	81
Expected future benefit payments for 2020	85
Expected future benefit payments for 2021	83
Expected future benefit payments for 2022	92
Expected future benefit payments for 2023-2027	453
Contributions expected to be paid in next fiscal year	126
United States \| OPEB
Benefits Expected to be Paid by the Company
Expected future benefit payments for 2018	25
Expected future benefit payments for 2019	25
Expected future benefit payments for 2020	25
Expected future benefit payments for 2021	25
Expected future benefit payments for 2022	24
Expected future benefit payments for 2023-2027	110
Contributions expected to be paid in next fiscal year	CAD 12

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Amount of benefits for defined benefit plan expected to be paid in five fiscal years after fifth fiscal year following latest fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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Amount of benefits for defined benefit plan expected to be paid in next fiscal year following latest fiscal year.

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Amount of contributions expected to be received by defined benefit plan from employer in next fiscal year following latest fiscal year.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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