Stock-Based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Schedule of stock-based compensation expense
Stock-based compensation	$ 2,063	$ 1,980
Stock options [Member]
Schedule of stock-based compensation expense
Stock-based compensation	554	472
RSAs and RSUs [Member]
Schedule of stock-based compensation expense
Stock-based compensation	461	452
Performance Stock Units (PSUs) [Member]
Schedule of stock-based compensation expense
Stock-based compensation	391	409
Performance Based Options (PBOs) [Member]
Schedule of stock-based compensation expense
Stock-based compensation	657	647
Research and development [Member]
Schedule of stock-based compensation expense
Stock-based compensation	388	435
Selling, General and Administrative Expenses [Member]
Schedule of stock-based compensation expense
Stock-based compensation	$ 1,675	$ 1,545