Stock-based compensation - Stock Options Outstanding and Exercisable (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2021 CAD ($) $ / shares shares
$15.14 - $38.17
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum (cad per share)	$ 15,140,000
Range of exercise prices, maximum (cad per share)	38,170,000
$38.18 - $46.65
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum (cad per share)	38,180,000
Range of exercise prices, maximum (cad per share)	46,650,000
$46.66 - $68.74
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum (cad per share)	46,660,000
Range of exercise prices, maximum (cad per share)	68,740,000
$68.75 - $103.98
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum (cad per share)	68,750,000
Range of exercise prices, maximum (cad per share)	103,980,000
Stock Option Plan
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Company's closing stock price (cad per share)	$ 90.98
Options outstanding, Number of options (in shares) | shares	7,392,188
Options outstanding, Weighted-average years to expiration	3 years 9 months 18 days
Options outstanding, Weighted-average exercise price (cad per share)	$ 53.36
Options outstanding, Aggregate intrinsic value | $	$ 278
Options exercisable, Number of options (in shares) | shares	3,643,205
Options exercisable, Weighted-average exercise price (cad per share)	$ 41.64
Options exercisable, Aggregate intrinsic value | $	$ 180
Number of stock options in-the-money (in shares) | shares	7,373,880
Weighted-average exercise price of stock options in-the-money (cad per share)	$ 53.26
Weighted-average years to expiration	3 years 1 month 6 days
Stock Option Plan | $15.14 - $38.17
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, Number of options (in shares) | shares	1,420,600
Options outstanding, Weighted-average years to expiration	2 years 8 months 12 days
Options outstanding, Weighted-average exercise price (cad per share)	$ 30.14
Options outstanding, Aggregate intrinsic value | $	$ 86
Options exercisable, Number of options (in shares) | shares	1,420,600
Options exercisable, Weighted-average exercise price (cad per share)	$ 30.14
Options exercisable, Aggregate intrinsic value | $	$ 87
Stock Option Plan | $38.18 - $46.65
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, Number of options (in shares) | shares	2,013,630
Options outstanding, Weighted-average years to expiration	2 years 4 months 24 days
Options outstanding, Weighted-average exercise price (cad per share)	$ 40.86
Options outstanding, Aggregate intrinsic value | $	$ 101
Options exercisable, Number of options (in shares) | shares	1,032,160
Options exercisable, Weighted-average exercise price (cad per share)	$ 42.64
Options exercisable, Aggregate intrinsic value | $	$ 50
Stock Option Plan | $46.66 - $68.74
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, Number of options (in shares) | shares	2,167,180
Options outstanding, Weighted-average years to expiration	4 years 1 month 6 days
Options outstanding, Weighted-average exercise price (cad per share)	$ 55.55
Options outstanding, Aggregate intrinsic value | $	$ 77
Options exercisable, Number of options (in shares) | shares	1,082,960
Options exercisable, Weighted-average exercise price (cad per share)	$ 52.89
Options exercisable, Aggregate intrinsic value | $	$ 41
Stock Option Plan | $68.75 - $103.98
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, Number of options (in shares) | shares	1,790,778
Options outstanding, Weighted-average years to expiration	5 years 10 months 24 days
Options outstanding, Weighted-average exercise price (cad per share)	$ 83.21
Options outstanding, Aggregate intrinsic value | $	$ 14
Options exercisable, Number of options (in shares) | shares	107,485
Options exercisable, Weighted-average exercise price (cad per share)	$ 70.48
Options exercisable, Aggregate intrinsic value | $	$ 2