Stock-based compensation - Stock Options Outstanding and Exercisable (Details) - Stock Option Plan $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023 CAD ($) $ / shares shares
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Company's closing stock price (cad per share)	$ 104.84
Options outstanding, number of options (in shares) | shares	6,471,932
Options outstanding, weighted-average years to expiration	3 years 3 months 18 days
Options outstanding, weighted-average exercise price (cad per share)	$ 71.03
Options outstanding, aggregate intrinsic value | $	$ 219
Options exercisable, number of options (in shares) | shares	4,168,829
Options exercisable, weighted-average exercise price (cad per share)	$ 58.20
Options exercisable, aggregate intrinsic value | $	$ 195
Number of stock options in-the-money (in shares) | shares	5,787,281
Weighted-average exercise price of stock options in-the-money (cad per share)	$ 66.96
Weighted-average years to expiration	2 years 3 months 18 days
Price Range One
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum (cad per share)	$ 30,940,000
Range of exercise prices, maximum (cad per share)	$ 50,190,000
Options outstanding, number of options (in shares) | shares	1,693,436
Options outstanding, weighted-average years to expiration	1 year 2 months 12 days
Options outstanding, weighted-average exercise price (cad per share)	$ 40.04
Options outstanding, aggregate intrinsic value | $	$ 110
Options exercisable, number of options (in shares) | shares	1,693,436
Options exercisable, weighted-average exercise price (cad per share)	$ 40.04
Options exercisable, aggregate intrinsic value | $	$ 110
Price Range Two
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum (cad per share)	$ 50,200,000
Range of exercise prices, maximum (cad per share)	$ 70,360,000
Options outstanding, number of options (in shares) | shares	1,395,999
Options outstanding, weighted-average years to expiration	2 years 3 months 18 days
Options outstanding, weighted-average exercise price (cad per share)	$ 58.53
Options outstanding, aggregate intrinsic value | $	$ 65
Options exercisable, number of options (in shares) | shares	1,284,814
Options exercisable, weighted-average exercise price (cad per share)	$ 57.53
Options exercisable, aggregate intrinsic value | $	$ 61
Price Range Three
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum (cad per share)	$ 70,370,000
Range of exercise prices, maximum (cad per share)	$ 94,270,000
Options outstanding, number of options (in shares) | shares	1,610,826
Options outstanding, weighted-average years to expiration	4 years
Options outstanding, weighted-average exercise price (cad per share)	$ 82.99
Options outstanding, aggregate intrinsic value | $	$ 35
Options exercisable, number of options (in shares) | shares	822,618
Options exercisable, weighted-average exercise price (cad per share)	$ 80.20
Options exercisable, aggregate intrinsic value | $	$ 20
Price Range Four
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum (cad per share)	$ 94,280,000
Range of exercise prices, maximum (cad per share)	$ 109,010,000.00
Options outstanding, number of options (in shares) | shares	1,771,671
Options outstanding, weighted-average years to expiration	5 years 3 months 18 days
Options outstanding, weighted-average exercise price (cad per share)	$ 99.62
Options outstanding, aggregate intrinsic value | $	$ 9
Options exercisable, number of options (in shares) | shares	367,961
Options exercisable, weighted-average exercise price (cad per share)	$ 94.94
Options exercisable, aggregate intrinsic value | $	$ 4