Stock-Based Compensation - Distribution of Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Expense [Line Items]
Stock-based compensation	$ 551	$ 443	$ 1,014	$ 733
Cost of Revenue
Share Based Compensation Expense [Line Items]
Stock-based compensation	28	9	46	22
Research and Development
Share Based Compensation Expense [Line Items]
Stock-based compensation	111	110	231	187
Selling, General and Administrative
Share Based Compensation Expense [Line Items]
Stock-based compensation	$ 412	$ 324	$ 737	$ 524