Income taxes - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Tax Disclosure [Abstract]
Income tax provision (benefit)	$ 221,000	$ 145,000	$ 312,000	$ 745,000
Unrecognized tax benefits, period change			0
Interest and penalties related to uncertain tax positions			$ 0