Income taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax [Line Items]
Operating Loss Carry forwards Expiration Period									2023
Income Tax Expense (Benefit)	$ (1,071)	$ 2,933	$ 2,962	$ 1,804	$ (603)	$ 663	$ 1,714	$ 1,170	$ 6,628	$ 2,944	$ (906)
Operating Loss Carryforwards	$ 5,400								$ 5,400
Operating Loss Carryforwards, Limitations On Use									All of the Companys remaining $5.4 million of NOLs are subject to annual limitations of $1.3 million.