Stock-Based Compensation Expense - Stock-Based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Stock-Based Compensation
Total stock-based compensation expense	$ 571	$ 333	$ 1,748	$ 806
Unrecognized compensation cost	5,800		$ 5,800
Period for recognition			2 years 8 months 12 days
Research and development.
Stock-Based Compensation
Total stock-based compensation expense	212	128	$ 635	307
General and administrative
Stock-Based Compensation
Total stock-based compensation expense	$ 359	$ 205	$ 1,113	$ 499

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Unrecognized cost of unvested share-based compensation awards.

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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