Stock-Based Compensation - Stock Options Outstanding and Exercisable (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020 CAD ($) $ / shares shares
$65.06 - $188.78
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum	$ 65.06
Range of exercise prices, maximum	188.78
$188.79 - $214.58
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum	188.79
Range of exercise prices, maximum	214.58
$214.59 - $261.88
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum	214.59
Range of exercise prices, maximum	261.88
$261.89 - $411.37
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum	261.89
Range of exercise prices, maximum	411.37
Stock Option Plan
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Company's closing stock price (CAD)	$ 441.53
Options outstanding, Number of options | shares	1,387,366
Options outstanding, Weighted-average years to expiration	4 years 2 months 12 days
Options outstanding, Weighted-average exercise price	$ 225.20
Options outstanding, Aggregate intrinsic value | $	$ 300
Options exercisable, Number of options (in shares) | shares	610,289
Options exercisable, Weighted-average exercise price	$ 177.65
Options exercisable, Aggregate intrinsic value | $	$ 161
Number of stock options in-the-money | shares	1,387,366
Weighted-average exercise price of stock options in-the-money	$ 225.20
Weighted-average years to expiration	3 years 7 months 6 days
Stock Option Plan | $65.06 - $188.78
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, Number of options | shares	342,773
Options outstanding, Weighted-average years to expiration	3 years 4 months 24 days
Options outstanding, Weighted-average exercise price	$ 141.26
Options outstanding, Aggregate intrinsic value | $	$ 103
Options exercisable, Number of options (in shares) | shares	342,773
Options exercisable, Weighted-average exercise price	$ 141.26
Options exercisable, Aggregate intrinsic value | $	$ 103
Stock Option Plan | $188.79 - $214.58
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, Number of options | shares	327,811
Options outstanding, Weighted-average years to expiration	3 years 1 month 6 days
Options outstanding, Weighted-average exercise price	$ 196.82
Options outstanding, Aggregate intrinsic value | $	$ 80
Options exercisable, Number of options (in shares) | shares	109,375
Options exercisable, Weighted-average exercise price	$ 203.83
Options exercisable, Aggregate intrinsic value | $	$ 26
Stock Option Plan | $214.59 - $261.88
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, Number of options | shares	394,953
Options outstanding, Weighted-average years to expiration	4 years 4 months 24 days
Options outstanding, Weighted-average exercise price	$ 244.17
Options outstanding, Aggregate intrinsic value | $	$ 78
Options exercisable, Number of options (in shares) | shares	134,845
Options exercisable, Weighted-average exercise price	$ 232.54
Options exercisable, Aggregate intrinsic value | $	$ 28
Stock Option Plan | $261.89 - $411.37
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, Number of options | shares	321,829
Options outstanding, Weighted-average years to expiration	5 years 9 months 18 days
Options outstanding, Weighted-average exercise price	$ 320.21
Options outstanding, Aggregate intrinsic value | $	$ 39
Options exercisable, Number of options (in shares) | shares	23,296
Options exercisable, Weighted-average exercise price	$ 272.56
Options exercisable, Aggregate intrinsic value | $	$ 4