Fair Value of Options Granted Calculated Using Weighted Average Assumptions (Detail) (USD $)	3 Months Ended
Mar. 31, 2012 Year	Mar. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted	46,950
Weighted average exercise price	$ 41.27
Weighted-average grant date fair value	$ 19.08
Assumptions:
Expected volatility	47.70%
Expected term (in years)	5.9
Risk-free interest rate	1.43%
Expected dividend yield

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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.

+ References

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- Definition

The period of time an equity-based award is expected to be outstanding. An equity-based award's expected term is generally determined based on, among other factors, the instrument's contractual term and the effects of employees' expected exercise and post-vesting employment termination behavior.

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-Name Accounting Standards Codification

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-Name Staff Accounting Bulletin (SAB)

-Number Topic 14

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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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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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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 price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on stock options awarded under the plan during the reporting period.

+ References

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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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