Stock-Based Payments - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 6,610	$ 4,419	$ 4,234
Stock options
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Unrecognized compensation costs	$ 14,800
Period in which compensation costs will be recognized	3 years
Performance-based stock options
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Unrecognized compensation costs	$ 1,000
Period in which compensation costs will be recognized	3 years 9 months 18 days
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 2,412	1,666	2,980
General and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 4,198	$ 2,753	$ 1,254