Stock Benefit Plans and Equity Transactions - Summary of Total Stock Based Compensation (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 1,612	$ 619
Cost of revenues
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	28	22
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	462	(116)
Sales, general and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 1,122	$ 713