Stock-based compensation - Weighted-Average Fair Value Assumptions (Details) - Stock Option Plan - $ / shares	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
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	3.88%	3.35%	1.62%
Expected stock price volatility	28.38%	28.44%	26.85%
Expected annual dividends (cad per share)	$ 0.76	$ 0.76	$ 0.76
Estimated forfeiture rate	3.12%	3.18%	3.01%
Weighted average grant date fair value of options granted during the year (cad per share)	$ 33.27	$ 29.79	$ 21.33