Trade accounts receivable - net (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning Balance	$ 722	$ 365	$ 335
Net additions charged to Operations	479	136	21
Deductions and Other	(23)	221	9
Ending Balance	$ 1,178	$ 722	$ 365