Stock-Based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Stock-based compensation expense
Stock-based compensation expense	$ 1,669	$ 1,729
Outstanding stock options
Stock-based compensation expense
Unrecognized stock-based compensation expense	$ 7,100
Weighted average period for recognition of shared-based compensation not yet recognized	2 years 15 days
Restricted stock, excluding performance-based vesting
Stock-based compensation expense
Unrecognized stock-based compensation expense	$ 200
Weighted average period expected to be recognized	5 months 9 days
RSUs
Stock-based compensation expense
Unrecognized stock-based compensation expense	$ 3,300
Weighted average period expected to be recognized	2 years 1 month 21 days
Research and development expenses
Stock-based compensation expense
Stock-based compensation expense	$ 640	820
General and administrative expenses
Stock-based compensation expense
Stock-based compensation expense	$ 1,029	$ 909

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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