Stock Options Activity (Detail) (USD $) In Millions, except Share data, unless otherwise specified	6 Months Ended
Jun. 30, 2012
Schedule Of Stock Options [Line Items]
Outstanding at January 1, 2012	353,452
Granted	95,899
Exercised	(39,562)
Forfeited	(15,734)
Outstanding at June 30, 2012	398,369
Exercisable at June 30, 2012	183,419
Outstanding at January 1, 2012	$ 27.65
Granted	$ 38.21
Exercised	$ 15.26
Forfeited	$ 35.50
Outstanding at June 30, 2012	$ 31.18
Exercisable at June 30, 2012	$ 25.39
Outstanding at June 30, 2012	7 years 5 months 1 day
Exercisable at June 30, 2012	5 years 9 months 15 days
Outstanding at June 30, 2012	$ 2.2
Exercisable at June 30, 2012	$ 1.8

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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

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

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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 remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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 can acquire the shares reserved for issuance on stock options awarded.

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

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