Goodwill and intangible assets - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Indefinite-lived Intangible Assets [Line Items]
Goodwill	$ 856	$ 856
Amortization of Intangible Assets	500	$ 500	$ 100
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	500
Finite-Lived Intangible Assets, Amortization Expense, Year Two	400
Finite-Lived Intangible Assets, Amortization Expense, Year Three	400
Finite-Lived Intangible Assets, Amortization Expense, Year Four	400
Finite-Lived Intangible Assets, Amortization Expense, Year Five	400
Finite-Lived Intangible Assets, Amortization Expense, after Year Five	1,100
Polar Technologies, LLC [Member]
Indefinite-lived Intangible Assets [Line Items]
Goodwill	400
Supplier [Member]
Indefinite-lived Intangible Assets [Line Items]
Goodwill	$ 400