Segment Information - Schedule of Product Revenue from Customers by Geographic Region (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues From External Customers And Long Lived Assets [Line Items]
Revenue	$ 8,667	$ 8,112	$ 22,190	$ 23,381
China
Revenues From External Customers And Long Lived Assets [Line Items]
Revenue	6,992	6,968	18,175	19,806
South Korea
Revenues From External Customers And Long Lived Assets [Line Items]
Revenue	705	705	2,018	2,156
Taiwan
Revenues From External Customers And Long Lived Assets [Line Items]
Revenue	589	277	933	650
Japan
Revenues From External Customers And Long Lived Assets [Line Items]
Revenue	370	150	1,047	714
Other
Revenues From External Customers And Long Lived Assets [Line Items]
Revenue	$ 11	$ 12	$ 17	$ 55