Pensions and other benefits - Amounts Recognized in Company's Consolidated Balance Sheets (Details) - CAD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]
Pension asset	$ 3,338	$ 3,101
Accounts payable and accrued liabilities	(48)	(43)
Pension and other benefit liabilities	(581)	(538)
Total amount recognized	2,709	2,520
Pensions
Defined Benefit Plan Disclosure [Line Items]
Pension asset	3,338	3,101
Accounts payable and accrued liabilities	(11)	(10)
Pension and other benefit liabilities	(161)	(165)
Total amount recognized	3,166	2,926
Other benefits
Defined Benefit Plan Disclosure [Line Items]
Pension asset	0	0
Accounts payable and accrued liabilities	(37)	(33)
Pension and other benefit liabilities	(420)	(373)
Total amount recognized	$ (457)	$ (406)