Summary of Weighted Average Assumptions and Fair Value of Options (Detail) (USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options granted	818,090	716,508	2,176,963
Weighted average exercise price	$ 22.95	$ 17.61	$ 16.01
Risk-free interest rate	1.69%	2.96%	2.65%
Expected volatility	24.11%	29.98%	30.00%
Expected term (years)	7 years 10 months 24 days	8 years 6 months	4 years 10 months 24 days
Dividend yield	1.75%	1.59%	1.79%
Weighted average fair value	$ 5.27	$ 5.78	$ 4.11

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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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

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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.

+ References

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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

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

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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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Weighted average price at which grantees can acquire the shares reserved for issuance on stock options awarded.

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