Pensions and other benefits - Narrative (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items]
Period of average market values for calculating net periodic benefit cost	5 years
Benefit obligation, discount rate	4.64%	5.01%	3.01%
Defined benefit plan with accumulated benefit obligation in excess of plan assets, accumulated benefit obligation	$ 327	$ 332
Defined benefit plan with accumulated benefit obligation in excess of plan assets, plan assets	$ 189	$ 186
Maximum value of underlying assets represented by financial derivatives, excluding currency forwards	30.00%
Solvency funded position hedged against interest rate risk	50.00%	45.00%
Company's common shares in fund assets (in shares)	354,530	570,074
Net cost of defined contribution plan	$ 14	$ 12	$ 13
Multi-employer benefit plan, Employer contributions	$ 4	$ 2	$ 3
US Dollar
Defined Benefit Plan Disclosure [Line Items]
Plans exposure to foreign currency risk percentage	41.00%	50.00%
Euro
Defined Benefit Plan Disclosure [Line Items]
Plans exposure to foreign currency risk percentage	7.00%	6.00%
Other Currencies
Defined Benefit Plan Disclosure [Line Items]
Plans exposure to foreign currency risk percentage	9.00%	10.00%
Public equity
Defined Benefit Plan Disclosure [Line Items]
Company's securities in fund assets	$ 37	$ 58
Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Company's securities in fund assets	$ 2	$ 5
Pensions
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation, discount rate	4.64%	5.01%	3.01%
Defined benefit pension plans accumulated benefit obligation	$ 10,155	$ 9,747