Earnings per share - Weighted Average Number of Shares Used in Computing Earnings Per Share (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Basic earnings per share	4,818,733	4,817,856	4,818,311	4,815,845
Effect of dilutive securities:
Stock options	18,050	2,158	14,105	2,413
Diluted earnings per share	4,836,783	4,820,014	4,832,416	4,818,258
Basic earnings per share	$ 0.05	$ 0.05	$ 0.15	$ 0.08
Diluted earnings per share	$ 0.05	$ 0.05	$ 0.15	$ 0.08