Stockholders' Equity - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Stock-based compensation	$ 1,201	$ 1,081	$ 1,060
Cost of Sales
Class of Stock [Line Items]
Stock-based compensation	154	143
Research and Development Expense
Class of Stock [Line Items]
Stock-based compensation	215	456	256
General and Administrative Expense
Class of Stock [Line Items]
Stock-based compensation	$ 832	$ 482	$ 804