Pensions and other benefits - Amounts Recognized in Company's Consolidated Balance Sheets (Details) - CAD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Pension asset	$ 2,317	$ 894
Pension and other benefit liabilities	(718)	(832)
Pensions
Defined Benefit Plan Disclosure [Line Items]
Pension asset	2,317	894
Accounts payable and accrued liabilities	(11)	(11)
Pension and other benefit liabilities	(252)	(317)
Total amount recognized	2,054	566
Other benefits
Defined Benefit Plan Disclosure [Line Items]
Pension asset	0	0
Accounts payable and accrued liabilities	(32)	(33)
Pension and other benefit liabilities	(466)	(515)
Total amount recognized	$ (498)	$ (548)