Earnings per share - Weighted Average Number of Shares Used in Computing Earnings Per Share (Detail) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Basic earnings per share	4,780,649	4,818,733	4,797,405	4,818,454
Effect of dilutive securities:
Stock options	15,359	9,552	13,724	12,601
Diluted earnings per share	4,796,008	4,828,285	4,811,129	4,831,055
Basic earnings per share	$ 0.06	$ 0.11	$ 0.20	$ 0.26
Diluted earnings per share	$ 0.06	$ 0.11	$ 0.20	$ 0.26