Summary of Stock Option Activity (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	9 Months Ended
Sep. 30, 2012
Options
Options, Outstanding, Beginning Balance	1,224,699
Options, Exercised	(771,809)
Options, Forfeited	(1,250)
Options, Outstanding and exercisable, Ending Balance	451,640
Weighted-Average Exercise Price
Weighted-Average Exercise Price, Outstanding, Beginning Balance	$ 21.64
Weighted-Average Exercise Price, Exercised	$ 20.88
Weighted-Average Exercise Price, Forfeited	$ 23.09
Weighted-Average Exercise Price, Outstanding and exercisable, Ending Balance	$ 22.95
Weighted-Average Remaining Contractual Term (years)
Weighted-Average Remaining Contractual Term (years), Outstanding and exercisable, Ending Balance	1 year 3 months 18 days
Aggregate Intrinsic Value
Aggregate Intrinsic Value, Outstanding and exercisable, Ending Balance	$ 6,030

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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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 weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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The number of shares reserved for issuance under stock option agreements awarded under the plan that validly exist and are outstanding as of the balance sheet date, including 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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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 that were terminated.

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

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