Stockholders' Equity - Schedule of Stock-Based Compensation Expense Recognized (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Employee share-based compensation expense	$ 8,633	$ 9,283	$ 6,297
Employee | Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Employee share-based compensation expense	3,613	3,923	2,034
Employee | General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Employee share-based compensation expense	4,879	5,029	3,810
Non-Employee | Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Employee share-based compensation expense	85	268	205
Non-Employee | General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Employee share-based compensation expense	$ 56	$ 63	$ 248