Revenues Based on Customer Location (Detail) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017		Jun. 30, 2018		Jun. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ 17,494	$ 20,575		$ 35,063		$ 41,862
United States
Segment Reporting Information [Line Items]
Revenues	2,121	3,622		3,828		4,925
Europe and Middle East
Segment Reporting Information [Line Items]
Revenues	2,713	2,253		5,166		6,184
Asia Pacific
Segment Reporting Information [Line Items]
Revenues	12,660	14,700		26,069		30,753
Germany
Segment Reporting Information [Line Items]
Revenues	1,895		[1]		[1]		[1]
China
Segment Reporting Information [Line Items]
Revenues	6,802	6,078		12,935		15,820
S. Korea
Segment Reporting Information [Line Items]
Revenues	2,877	5,027		5,679		8,776
Japan
Segment Reporting Information [Line Items]
Revenues	$ 2,523	$ 2,692		$ 5,147			[1]

[1] Less than 10%