Stock-based compensation - Stock Options Outstanding and Exercisable (Details) - Stock Option Plan $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2024 CAD ($) $ / shares shares
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Company's closing stock price (cad per share)	$ 104.08
Options outstanding, number of options (in shares) | shares	5,734,600
Options outstanding, weighted-average years to expiration	3 years 2 months 12 days
Options outstanding, weighted-average exercise price (cad per share)	$ 86.59
Options outstanding, aggregate intrinsic value | $	$ 116
Options exercisable, number of options (in shares) | shares	3,690,970
Options exercisable, weighted-average exercise price (cad per share)	$ 74.40
Options exercisable, aggregate intrinsic value | $	$ 110
Number of stock options in-the-money (in shares) | shares	4,206,139
Weighted-average exercise price of stock options in-the-money (cad per share)	$ 74.40
Weighted-average years to expiration	2 years 1 month 6 days
Price Range One
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum (cad per share)	$ 33.15
Range of exercise prices, maximum (cad per share)	$ 68.94
Options outstanding, number of options (in shares) | shares	1,371,256
Options outstanding, weighted-average years to expiration	1 year
Options outstanding, weighted-average exercise price (cad per share)	$ 48.49
Options outstanding, aggregate intrinsic value | $	$ 77
Options exercisable, number of options (in shares) | shares	1,371,256
Options exercisable, weighted-average exercise price (cad per share)	$ 48.49
Options exercisable, aggregate intrinsic value | $	$ 76
Price Range Two
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum (cad per share)	$ 68.95
Range of exercise prices, maximum (cad per share)	$ 91.73
Options outstanding, number of options (in shares) | shares	1,479,444
Options outstanding, weighted-average years to expiration	2 years 8 months 12 days
Options outstanding, weighted-average exercise price (cad per share)	$ 80.21
Options outstanding, aggregate intrinsic value | $	$ 35
Options exercisable, number of options (in shares) | shares	1,222,834
Options exercisable, weighted-average exercise price (cad per share)	$ 78.30
Options exercisable, aggregate intrinsic value | $	$ 32
Price Range Three
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum (cad per share)	$ 91.74
Range of exercise prices, maximum (cad per share)	$ 105.49
Options outstanding, number of options (in shares) | shares	1,357,794
Options outstanding, weighted-average years to expiration	3 years 6 months
Options outstanding, weighted-average exercise price (cad per share)	$ 100.95
Options outstanding, aggregate intrinsic value | $	$ 4
Options exercisable, number of options (in shares) | shares	906,268
Options exercisable, weighted-average exercise price (cad per share)	$ 100.77
Options exercisable, aggregate intrinsic value | $	$ 2
Price Range Four
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum (cad per share)	$ 105.50
Range of exercise prices, maximum (cad per share)	$ 121.28
Options outstanding, number of options (in shares) | shares	1,526,106
Options outstanding, weighted-average years to expiration	5 years 7 months 6 days
Options outstanding, weighted-average exercise price (cad per share)	$ 114.24
Options outstanding, aggregate intrinsic value | $	$ 0
Options exercisable, number of options (in shares) | shares	190,612
Options exercisable, weighted-average exercise price (cad per share)	$ 110.51
Options exercisable, aggregate intrinsic value | $	$ 0