Stock-Based Compensation - Stock Option Plan (Detail) - CAD CAD / shares in Units, CAD in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of options, Options outstanding (in shares)
Outstanding, January 1	2,450,082
Granted	369,980
Exercised	(536,291)
Forfeited	(803,796)
Expired	(700)
Outstanding, December 31	1,479,275	2,450,082
Vested or expected to vest at December 31	1,419,564
Exercisable, December 31	796,348
Number of options, Nonvested options (in shares)
Outstanding (non-vested) January 1	850,167
Granted	369,980
Vested	(328,265)
Forfeited (non-vested)	(208,955)
Outstanding (non-vested) December 31	682,927	850,167
Weighted average exercise price, Options outstanding
Outstanding, January 1 (in dollars per share)	CAD 121.95
New options granted (in dollars per share)	193.13
Exercised (in dollars per share)	84.93
Forfeited (in dollars per share)	120.97
Expired (in dollars per share)	62.56
Outstanding at December 31 (in dollar per share)	150.64	CAD 121.95
Vested or expected to vest as at December 31 (in dollars per share)	149.05
Exercisable at December 31 (in dollars per share)	119.32
Weighted average grant date fair value, Nonvested options
Outstanding (non-vested), January 1 (in dollars per share)	44.49
Weighted average grant date fair value of options granted during the year (in dollars per share)	45.78	39.01	CAD 55.28
Vested (in dollars per share)	42.97
Forfeited (in dollars per share)	47.21
Outstanding (non-vested), December 31 (in dollars per share)	CAD 45.46	CAD 44.49
Weighted average remaining term of vested or expected to vest options	7 years
Aggregate intrinsic value of vested or expected to vest options	CAD 114