Summary of stock option activity (Detail) (USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Options Outstanding
Outstanding at beginning of period	206,946	227,518	255,267
Granted	0	0	0
Exercised	0	0	0
Forfeited	0	0	0
Expired	(45,960)	(20,572)	(27,749)
Outstanding at end of period	160,986	206,946	227,518
Share Based Compensation Arrangement By Share Based Payment Award Options Additional Disclosures Abstract
Outstanding at beginning of period	$ 207.83	$ 206.71	$ 206.44
Granted	$ 0	$ 0	$ 0
Exercised	$ 0	$ 0	$ 0
Forfeited	$ 0	$ 0	$ 0
Share Based Compensation Arrangement By Share Based Payment Award Options Expirations In Period Weighted Average Exercise Price	$ 155.68	$ 195.48	$ 204.27
Outstanding at end of period	$ 222.71	$ 207.83	$ 206.71

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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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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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Gross 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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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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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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