Stock-based compensation - Stock Options Outstanding and Exercisable (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2022 CAD ($) $ / shares shares
$23.84 - $41.51
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum (cad per share)	$ 23,840,000
Range of exercise prices, maximum (cad per share)	41,510,000
$41.52 - $55.48
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum (cad per share)	41,520,000
Range of exercise prices, maximum (cad per share)	55,480,000
$55.49 - $90.81
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum (cad per share)	55,490,000
Range of exercise prices, maximum (cad per share)	90,810,000
$90.82 - $111.52
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum (cad per share)	90,820,000
Range of exercise prices, maximum (cad per share)	111,520,000
Stock Option Plan
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Company's closing stock price (cad per share)	$ 100.95
Options outstanding, Number of options (in shares) | shares	7,353,133
Options outstanding, Weighted-average years to expiration	3 years 3 months 18 days
Options outstanding, Weighted-average exercise price (cad per share)	$ 61.69
Options outstanding, Aggregate intrinsic value | $	$ 289
Options exercisable, Number of options (in shares) | shares	4,756,125
Options exercisable, Weighted-average exercise price (cad per share)	$ 49.02
Options exercisable, Aggregate intrinsic value | $	$ 247
Number of stock options in-the-money (in shares) | shares	7,339,374
Weighted-average exercise price of stock options in-the-money (cad per share)	$ 61.60
Weighted-average years to expiration	2 years 4 months 24 days
Stock Option Plan | $23.84 - $41.51
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, Number of options (in shares) | shares	2,105,506
Options outstanding, Weighted-average years to expiration	1 year 7 months 6 days
Options outstanding, Weighted-average exercise price (cad per share)	$ 36.62
Options outstanding, Aggregate intrinsic value | $	$ 135
Options exercisable, Number of options (in shares) | shares	2,105,506
Options exercisable, Weighted-average exercise price (cad per share)	$ 36.62
Options exercisable, Aggregate intrinsic value | $	$ 135
Stock Option Plan | $41.52 - $55.48
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, Number of options (in shares) | shares	1,655,284
Options outstanding, Weighted-average years to expiration	2 years 3 months 18 days
Options outstanding, Weighted-average exercise price (cad per share)	$ 49.09
Options outstanding, Aggregate intrinsic value | $	$ 86
Options exercisable, Number of options (in shares) | shares	1,526,734
Options exercisable, Weighted-average exercise price (cad per share)	$ 48.64
Options exercisable, Aggregate intrinsic value | $	$ 80
Stock Option Plan | $55.49 - $90.81
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, Number of options (in shares) | shares	1,795,994
Options outstanding, Weighted-average years to expiration	4 years
Options outstanding, Weighted-average exercise price (cad per share)	$ 70.03
Options outstanding, Aggregate intrinsic value | $	$ 56
Options exercisable, Number of options (in shares) | shares	883,395
Options exercisable, Weighted-average exercise price (cad per share)	$ 66.85
Options exercisable, Aggregate intrinsic value | $	$ 30
Stock Option Plan | $90.82 - $111.52
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, Number of options (in shares) | shares	1,796,349
Options outstanding, Weighted-average years to expiration	5 years 7 months 6 days
Options outstanding, Weighted-average exercise price (cad per share)	$ 94.35
Options outstanding, Aggregate intrinsic value | $	$ 12
Options exercisable, Number of options (in shares) | shares	240,490
Options exercisable, Weighted-average exercise price (cad per share)	$ 94.49
Options exercisable, Aggregate intrinsic value | $	$ 2