Stock-Based Compensation - Weighted-Average Fair Value Assumptions (Detail) - CAD		9 Months Ended	12 Months Ended
Apr. 20, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Expected option life (years)			5 years 3 months	5 years 3 months	5 years 11 months 23 days
Risk-free interest rate			1.21%	1.10%	1.66%
Expected stock price volatility			27.00%	26.00%	29.00%
Expected annual dividends per share (in dollars per share)	CAD 0.50		CAD 1.40	CAD 1.40	CAD 1.40
Estimated forfeiture rate			2.00%	1.20%	1.20%
Weighted average grant date fair value of options granted during the year (in dollars per share)			CAD 39.01	CAD 55.28	CAD 48.88
Annual Dividends		CAD 2.00

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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- Definition

Amount of paid and unpaid cash dividends declared and applicable to holders of share-based compensation, for example, but not limited to, non-vested shares, stock options, or restricted stock units.

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No definition available.

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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.

+ References

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=6415400&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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