PENSION AND POST-RETIREMENT AND POST-EMPLOYMENT BENEFITS - Summary of Benefit Obligations and Plan Assets (Details) - CAD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Pension Benefits
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Other assets	$ 11	$ 12
Deferred pension assets	610	647
Accrued liabilities	0	0
Post-retirement and post-employment benefit liability	0	0
Net (funded) unfunded status	(621)	(659)
Post-Retirement and Post-Employment Benefits
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Other assets	0	0
Deferred pension assets	0	0
Accrued liabilities	78	76
Post-retirement and post-employment benefit liability	1,672	1,590
Net (funded) unfunded status	$ 1,750	$ 1,666

Defined Benefit Plan, Deferred Pension Assets

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Defined Benefit Plan, Other Pension Assets

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Pension Plans, Postretirement and Other Employee Benefits [Line Items]

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Amount of asset (liability), recognized in statement of financial position, for defined benefit pension and other postretirement plans.

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Amount of liability, recognized in statement of financial position, for defined benefit other postretirement plan, classified as noncurrent. Excludes pension plan.

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Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans, classified as current.

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