PENSION AND OTHER POSTRETIREMENT BENEFITS - EXPECTED BENEFIT PAYMENTS AND EMPLOYER CONTRIBUTIONS (Details) $ in Millions	Dec. 31, 2020 CAD ($)
Canada \| Pension
Benefits Expected to be Paid by the Company
2021	$ 185
2022	189
2023	194
2024	198
2025	203
2026-2030	1,078
Contributions expected to be paid in next fiscal year	102
Canada \| OPEB
Benefits Expected to be Paid by the Company
2021	12
2022	13
2023	13
2024	13
2025	13
2026-2030	71
Contributions expected to be paid in next fiscal year	12
United States \| Pension
Benefits Expected to be Paid by the Company
2021	139
2022	76
2023	75
2024	75
2025	74
2026-2030	353
Contributions expected to be paid in next fiscal year	49
United States \| OPEB
Benefits Expected to be Paid by the Company
2021	19
2022	18
2023	17
2024	16
2025	15
2026-2030	66
Contributions expected to be paid in next fiscal year	$ 7

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

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4587-114921

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