Summary of stock option activity (Detail) (USD $)	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Options Outstanding
Outstanding at beginning of period			160,986	206,946	206,946
Granted			0		0
Exercised	0	0	0	0	0
Forfeited			0		0
Expired			(59,231)		(45,960)
Outstanding at end of period	101,755		101,755		160,986
Share Based Compensation Arrangement By Share Based Payment Award Options Additional Disclosures Abstract
Outstanding at beginning of period			$ 222.71	$ 207.83	$ 207.83
Granted			$ 0		$ 0
Exercised			$ 0		$ 0
Forfeited			$ 0		$ 0
Share Based Compensation Arrangement By Share Based Payment Award Options Expirations In Period Weighted Average Exercise Price			$ 170.10		$ 155.68
Outstanding at end of period	$ 253.34		$ 253.34		$ 222.71

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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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