Stock-Based Compensation - Stock Options Outstanding and Exercisable (Detail) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018 CAD ($) $ / shares shares
Range of exercise prices from $36.29 - $122.76 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of option exercise prices, minimum	$ 36.29
Range of option exercise prices, maximum	122.76
Range of exercise prices from $122.77 - $200.75 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of option exercise prices, minimum	122.77
Range of option exercise prices, maximum	200.75
Range of exercise prices from $200.76 - $217.68 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of option exercise prices, minimum	200.76
Range of option exercise prices, maximum	217.68
Range of exercise prices from $217.69 - $272.92 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of option exercise prices, minimum	217.69
Range of option exercise prices, maximum	$ 272.92
Employee Stock Option [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options outstanding, Number of options (in shares) | shares	1,533,598
Options outstanding, Weighted average years to expiration	5 years 2 months 12 days
Options outstanding, Weighted average exercise price (in dollars per share)	$ 176.02
Options outstanding, Aggregate intrinsic value | $	$ 104
Options exercisable, Number of options (in shares) | shares	819,496
Options exercisable, Weighted average exercise price (in dollars per share)	$ 140.54
Options outstanding, Aggregate intrinsic value | $	$ 83
Stock options in-the-money (in shares) | shares	1,342,215
Stock options in-the-money weighted average exercise price (in dollars per share)	$ 164.62
Stock options weighted average contractual Term	4 years 6 months
Employee Stock Option [Member] | Range of exercise prices from $36.29 - $122.76 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options outstanding, Number of options (in shares) | shares	410,049
Options outstanding, Weighted average years to expiration	3 years 3 months 18 days
Options outstanding, Weighted average exercise price (in dollars per share)	$ 93.51
Options outstanding, Aggregate intrinsic value | $	$ 61
Options exercisable, Number of options (in shares) | shares	410,049
Options exercisable, Weighted average exercise price (in dollars per share)	$ 93.51
Options outstanding, Aggregate intrinsic value | $	$ 61
Employee Stock Option [Member] | Range of exercise prices from $122.77 - $200.75 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options outstanding, Number of options (in shares) | shares	339,686
Options outstanding, Weighted average years to expiration	6 years 1 month 6 days
Options outstanding, Weighted average exercise price (in dollars per share)	$ 164.47
Options outstanding, Aggregate intrinsic value | $	$ 26
Options exercisable, Number of options (in shares) | shares	232,206
Options exercisable, Weighted average exercise price (in dollars per share)	$ 164.49
Options outstanding, Aggregate intrinsic value | $	$ 18
Employee Stock Option [Member] | Range of exercise prices from $200.76 - $217.68 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options outstanding, Number of options (in shares) | shares	388,527
Options outstanding, Weighted average years to expiration	5 years 1 month 6 days
Options outstanding, Weighted average exercise price (in dollars per share)	$ 205.80
Options outstanding, Aggregate intrinsic value | $	$ 14
Options exercisable, Number of options (in shares) | shares	88,013
Options exercisable, Weighted average exercise price (in dollars per share)	$ 207.63
Options outstanding, Aggregate intrinsic value | $	$ 3
Employee Stock Option [Member] | Range of exercise prices from $217.69 - $272.92 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options outstanding, Number of options (in shares) | shares	395,336
Options outstanding, Weighted average years to expiration	6 years 2 months 12 days
Options outstanding, Weighted average exercise price (in dollars per share)	$ 242.25
Options outstanding, Aggregate intrinsic value | $	$ 3
Options exercisable, Number of options (in shares) | shares	89,228
Options exercisable, Weighted average exercise price (in dollars per share)	$ 228.12
Options outstanding, Aggregate intrinsic value | $	$ 1