Stock-Based Compensation - Stock Options Outstanding and Exercisable (Detail) CAD / shares in Units, CAD in Millions	12 Months Ended
	Dec. 31, 2016 CAD CAD / shares shares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options outstanding, Number of options (in shares) | shares	2,450,082
Options outstanding, Weighted average years to expiration	6 years 2 months 30 days
Options outstanding, Weighted average exercise price (in dollars per share)	CAD 121.94
Options outstanding, Aggregate intrinsic value | CAD	CAD 171
Options exercisable, Number of options (in shares) | shares	1,599,915
Options exercisable, Weighted average exercise price (in dollars per share)	CAD 93.79
Options outstanding, Aggregate intrinsic value | CAD	CAD 156
Stock options in-the-money (in shares) | shares	2,140,692
Stock options in-the-money weighted average exercise price (in dollars per share)	CAD 107.27
Stock options weighted average contractual Term	5 years 25 days
Range of exercise prices from $36.29 - $72.54 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of option exercise prices, minimum	CAD 36.29
Range of option exercise prices, maximum	CAD 72.54
Options outstanding, Number of options (in shares) | shares	304,025
Options outstanding, Weighted average years to expiration	2 years 3 months 30 days
Options outstanding, Weighted average exercise price (in dollars per share)	CAD 60.59
Options outstanding, Aggregate intrinsic value | CAD	CAD 40
Options exercisable, Number of options (in shares) | shares	304,025
Options exercisable, Weighted average exercise price (in dollars per share)	CAD 60.59
Options outstanding, Aggregate intrinsic value | CAD	CAD 40
Range of exercise prices from $72.55 - $86.71 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of option exercise prices, minimum	CAD 72.55
Range of option exercise prices, maximum	CAD 86.71
Options outstanding, Number of options (in shares) | shares	747,445
Options outstanding, Weighted average years to expiration	5 years 4 months 30 days
Options outstanding, Weighted average exercise price (in dollars per share)	CAD 73.69
Options outstanding, Aggregate intrinsic value | CAD	CAD 88
Options exercisable, Number of options (in shares) | shares	747,445
Options exercisable, Weighted average exercise price (in dollars per share)	CAD 73.69
Options outstanding, Aggregate intrinsic value | CAD	CAD 88
Range of exercise prices from $74.56 - $161.38 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of option exercise prices, minimum	CAD 86.72
Range of option exercise prices, maximum	CAD 161.38
Options outstanding, Number of options (in shares) | shares	678,416
Options outstanding, Weighted average years to expiration	7 years
Options outstanding, Weighted average exercise price (in dollars per share)	CAD 127.62
Options outstanding, Aggregate intrinsic value | CAD	CAD 44
Options exercisable, Number of options (in shares) | shares	347,061
Options exercisable, Weighted average exercise price (in dollars per share)	CAD 111.34
Options outstanding, Aggregate intrinsic value | CAD	CAD 28
Range of exercise prices from $161.38 - $236.50 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of option exercise prices, minimum	CAD 161.39
Range of option exercise prices, maximum	CAD 236.50
Options outstanding, Number of options (in shares) | shares	720,196
Options outstanding, Weighted average years to expiration	7 years 8 months 300 days
Options outstanding, Weighted average exercise price (in dollars per share)	CAD 192.56
Options outstanding, Aggregate intrinsic value | CAD	CAD (1)
Options exercisable, Number of options (in shares) | shares	201,384
Options exercisable, Weighted average exercise price (in dollars per share)	CAD 188.23
Options outstanding, Aggregate intrinsic value | CAD	CAD 0