Stock-based compensation - Weighted-Average Fair Value Assumptions (Details) - Stock Option Plan - $ / shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected option life (years)	4 years 9 months	4 years 9 months	4 years 9 months
Risk-free interest rate	3.35%	1.62%	0.53%
Expected stock price volatility	28.44%	26.85%	27.14%
Expected annual dividends (cad per share)	$ 0.76	$ 0.76	$ 0.76
Estimated forfeiture rate	3.18%	3.01%	2.62%
Weighted average grant date fair value of options granted during the year (cad per share)	$ 29.79	$ 21.33	$ 19.06