Pensions and other benefits - Narrative (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]
Period of average market values for calculating net periodic benefit cost	5 years
Benefit obligation, discount rate	4.68%	4.64%	5.01%
Defined benefit plan with accumulated benefit obligation in excess of plan assets, accumulated benefit obligation	$ 159	$ 327
Defined benefit plan with accumulated benefit obligation in excess of plan assets, plan assets	$ 21	$ 189
Maximum value of underlying assets represented by financial derivatives, excluding currency forwards	30.00%
Solvency funded position hedged against interest rate risk	51.00%	50.00%
Company's common shares in fund assets (in shares)	322,733	354,530
Net cost of defined contribution plan	$ 16	$ 14	$ 12
Defined contribution plan, estimated employer contribution next fiscal year	17
Multi-employer benefit plan, Employer contributions	$ 3	$ 4	$ 2
US Dollar
Defined Benefit Plan Disclosure [Line Items]
Plans exposure to foreign currency risk percentage	39.00%	41.00%
Euro
Defined Benefit Plan Disclosure [Line Items]
Plans exposure to foreign currency risk percentage	7.00%	7.00%
Other Currencies
Defined Benefit Plan Disclosure [Line Items]
Plans exposure to foreign currency risk percentage	5.00%	9.00%
Public equity
Defined Benefit Plan Disclosure [Line Items]
Company's securities in fund assets	$ 34	$ 37
Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Company's securities in fund assets	2	2
Real estate
Defined Benefit Plan Disclosure [Line Items]
Fund value subject to redemption	435	480
Pensions
Defined Benefit Plan Disclosure [Line Items]
Expected future employer contributions, next fiscal year	13
Defined benefit pension plans accumulated benefit obligation	10,006	$ 10,155
Other benefits
Defined Benefit Plan Disclosure [Line Items]
Expected future employer contributions, next fiscal year	$ 35