Employee Benefit Plans - Stock Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-based compensation expense	$ 2,192	$ 3,797	$ 3,952
Cost of sales
Stock-based compensation expense	157	354	294
Research and development
Stock-based compensation expense	387	845	867
Selling, general and administrative
Stock-based compensation expense	$ 1,648	$ 2,598	$ 2,791