Summary of Option Activity (Detail) (USD $)	6 Months Ended
Jun. 30, 2013
Number of Shares
Outstanding at January 1	570,840
Granted	237,500
Exercised	14,715
Expired/Forfeited
Outstanding June 30	793,625
Exercisable at June 30	453,000
Weighted average exercise price per share
Outstanding at January 1	$ 41.05
Granted	$ 44.42
Exercised	$ 34.19
Expired/Forfeited
Outstanding June 30	$ 42.19
Exercisable at June 30	$ 41.77
Aggregate intrinsic value
Outstanding at January 1	$ 2,228,639
Granted	9,500
Exercised	131,885
Outstanding June 30	2,844,076
Exercisable at June 30	$ 2,257,858

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Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Intrinsic Value

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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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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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For presentations that combine terminations, 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 or that expired.

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Weighted average price of options that were either forfeited or expired.

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

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-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 per share amount at which grantees can acquire shares of common stock by exercise of options.

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

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