Earnings per share - Weighted Average Number of Shares Used in Computing Earnings Per Share (Detail) - $ / shares	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Basic earnings per share	4,802,032	4,818,733	4,805,922	4,818,311
Effect of dilutive securities:
Stock options	17,290	18,050	13,565	14,105
Diluted earnings per share	4,819,322	4,836,783	4,819,487	4,832,416
Basic earnings per share	$ 0.06	$ 0.05	$ 0.14	$ 0.15
Diluted earnings per share	$ 0.06	$ 0.05	$ 0.14	$ 0.15