Stockholders' Equity - Allocation of Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Stock-Based Compensation Expense
Total stock-based compensation expense	$ 967	$ 940	$ 2,727	$ 2,607
Research and development
Stock-Based Compensation Expense
Total stock-based compensation expense	309	299	889	796
General and administrative
Stock-Based Compensation Expense
Total stock-based compensation expense	$ 658	$ 641	$ 1,838	$ 1,811