Employee retirement benefits - Fair Value of Pension Plan Assets Including Level Within Fair Value Hierarchy (Detail) - CAD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	$ 8,582	$ 8,553
Equity securities, Canadian
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
Equity Securities, Non-Canadian
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	1,466	2,584
Debt Securities - Canadian, Corporate
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	1,693	1,220
Debt Securities - Canadian, Government
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	4,950	4,400
Debt Securities - Canadian, Asset Backed
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	4	4
Debt Securities - Canadian, Other
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	45	18
Equities – Venture capital
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	167	134
Real Estate
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	207	154
Cash
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	50	39
Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	18	3
Level 1 \| Equity securities, Canadian
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 1 \| Equity Securities, Non-Canadian
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 1 \| Debt Securities - Canadian, Corporate
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 1 \| Debt Securities - Canadian, Government
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 1 \| Debt Securities - Canadian, Asset Backed
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 1 \| Debt Securities - Canadian, Other
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 1 \| Equities – Venture capital
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 1 \| Real Estate
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 1 \| Cash
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	18	3
Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 2 \| Equity securities, Canadian
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 2 \| Equity Securities, Non-Canadian
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 2 \| Debt Securities - Canadian, Corporate
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 2 \| Debt Securities - Canadian, Government
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 2 \| Debt Securities - Canadian, Asset Backed
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 2 \| Debt Securities - Canadian, Other
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 2 \| Equities – Venture capital
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 2 \| Real Estate
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 2 \| Cash
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 3 \| Equity securities, Canadian
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 3 \| Equity Securities, Non-Canadian
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 3 \| Debt Securities - Canadian, Corporate
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 3 \| Debt Securities - Canadian, Government
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 3 \| Debt Securities - Canadian, Asset Backed
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 3 \| Debt Securities - Canadian, Other
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 3 \| Equities – Venture capital
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 3 \| Real Estate
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Level 3 \| Cash
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value
Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	8,564	8,550
Net Asset Value \| Equity securities, Canadian
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
Net Asset Value \| Equity Securities, Non-Canadian
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	1,466	2,584
Net Asset Value \| Debt Securities - Canadian, Corporate
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	1,693	1,220
Net Asset Value \| Debt Securities - Canadian, Government
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	4,950	4,400
Net Asset Value \| Debt Securities - Canadian, Asset Backed
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	4	4
Net Asset Value \| Debt Securities - Canadian, Other
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	45	18
Net Asset Value \| Equities – Venture capital
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	167	134
Net Asset Value \| Real Estate
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	207	154
Net Asset Value \| Cash
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	$ 32	$ 36