Summary of Stock Option Activity (Detail) (USD $)	9 Months Ended
Sep. 30, 2012
Directors Stock Option Plan
Number of Shares
Outstanding at December 31, 2011	102,000
Granted	13,500
Exercised	(3,000)
Expired	(1,500)
Outstanding at September 30, 2012	111,000
Weighted Average Exercise Price
Outstanding at December 31, 2011	$ 21.00
Granted	$ 18.76
Exercised	$ 15.00
Expired	$ 15.00
Outstanding at September 30, 2012	$ 20.97
Employee Stock Option
Number of Shares
Outstanding at December 31, 2011	135,500
Exercised	(14,500)
Expired	(13,000)
Outstanding at September 30, 2012	108,000
Weighted Average Exercise Price
Outstanding at December 31, 2011	$ 19.96
Exercised	$ 14.65
Expired	$ 18.09
Outstanding at September 30, 2012	$ 20.90

Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

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-Name Accounting Standards Codification

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

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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 of the plan that expired.

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

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