Stock Options Outstanding (Detail) (USD $)	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Number of Shares
Beginning balance	157,500	191,000	231,750
Granted
Exercised	(52,250)	(29,000)	(40,750)
Expired or forfeited		(4,500)
Ending balance	105,250	157,500	191,000
Number of options exercisable	105,250	157,500	191,000
Average Exercise Price
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price, Beginning of Period	$ 27.13	$ 26.21	$ 25.62
Granted
Exercised	$ 22.35	$ 21.12	$ 23.16
Expired or forfeited		$ 26.69
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price, End of Period	$ 29.50	$ 27.13	$ 26.21
Number of options exercisable	$ 29.50	$ 27.13	$ 26.21

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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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The weighted average price at which option holders acquired shares when converting their stock options into shares under the plan during the reporting period.

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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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The combined weighted average exercise price associated with stock options during the period that have either lapsed or been forfeited.

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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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The weighted average price as of the beginning of the year at which grantees can acquire the shares reserved for issuance under the stock option plan.

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