Trade accounts receivable - net (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning Balance	$ 365	$ 335	$ 244
Net additions charged to Operations	136	21	99
Deductions and Other	221	9	(8)
Ending Balance	$ 722	$ 365	$ 335