Stock-Based Compensation - Weighted-Average Fair Value Assumptions (Detail) - CAD		12 Months Ended
May 10, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Expected option life (years)		5 years 5 months 23 days	5 years 3 months	5 years 3 months
Risk-free interest rate		1.85%	1.21%	1.10%
Expected stock price volatility		27.00%	27.00%	26.00%
Expected annual dividends per share (in dollars per share)	CAD 0.5625	CAD 2.0010	CAD 1.4000	CAD 1.4000
Share based Compensation Arrangement by Share based Payment Award, Fair Value Assumptions, Estimated Forfeiture Rate		2.80%	2.00%	1.20%
Weighted average grant date fair value of options granted during the year (in dollars per share)		CAD 45.78	CAD 39.01	CAD 55.28
Expected Annual Dividends		CAD 2.2500

Company estimated forfeitures based on past experience.

No definition available.

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- Definition

Share Based Compensation Arrangement By Share Based Payment Award, Fair Value Assumptions, Expected Dividends Per Share

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No definition available.

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No definition available.

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- Definition

The estimated amount of dividends to be paid to holders of the underlying shares (expected dividends) over the option's term. Dividends are taken into account because payment of dividends to shareholders reduces the fair value of the underlying shares, and option holders generally do not receive dividends.

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- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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- Definition

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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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

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