Stock-Based Compensation - Stock Option Plan (Detail) - Employee Stock Option [Member] - CAD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of options, Options outstanding (in shares)
Outstanding, January 1	1,479,275
Granted	282,125
Exercised	(200,552)
Forfeited	(26,650)
Expired	(600)
Outstanding, December 31	1,533,598	1,479,275
Vested or expected to vest at December 31	1,500,135
Exercisable, December 31	819,496
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Outstanding, January 1 (in dollars per share)	$ 150.64
Granted (in dollars per share)	248.54
Exercised (in dollars per share)	118.86
Forfeited (in dollars per share)	211.80
Expired (in dollars per share)	71.69
Outstanding at December 31 (in dollar per share)	176.02	$ 150.64
Vested or expected to vest as at December 31 (in dollars per share)	174.77
Exercisable at December 31 (in dollars per share)	$ 140.54
Number of options, Nonvested options (in shares)
Outstanding (non-vested) January 1	682,927
Granted	282,125
Vested	(226,623)
Forfeited (non-vested)	(24,327)
Outstanding (non-vested) December 31	714,102	682,927
Weighted average grant date fair value, Nonvested options
Outstanding (non-vested), January 1 (in dollars per share)	$ 45.46
Weighted average grant date fair value of options granted during the year (in dollars per share)	55.63	$ 45.78	$ 39.01
Vested (in dollars per share)	46.94
Forfeited (in dollars per share)	47.65
Outstanding (non-vested), December 31 (in dollars per share)	$ 48.94	$ 45.46
Weighted average remaining term of vested or expected to vest options	7 years
Aggregate intrinsic value of vested or expected to vest options	$ 104

Share based Compensation Arrangement by Share based Payment Award, Weighted Average Grant Date Fair Value [Abstract]

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest exercisable options that may be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options of the plan that expired.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Number of non-vested options outstanding.

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Weighted average grant-date fair value of non-vested options forfeited.

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Weighted average grant-date fair value of non-vested options outstanding.

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Weighted average remaining contractual term for fully vested and expected to vest exercisable or convertible options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of options vested.

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Weighted average grant-date fair value of options vested.

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Number of share options (or share units) exercised during the current period.

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