Earnings per share - Weighted Average Number of Shares Used in Computing Earnings Per Share (Detail) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Basic earnings per share	4,818,377	4,816,859
Effect of dilutive securities:
Stock options	11,204	2,484
Diluted earnings per share	4,829,581	4,819,343
Basic earnings per share	$ 0.38us-gaap_EarningsPerShareBasic	$ 0.25us-gaap_EarningsPerShareBasic
Diluted earnings per share	$ 0.38us-gaap_EarningsPerShareDiluted	$ 0.25us-gaap_EarningsPerShareDiluted

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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