Stock-Based Compensation - Accounting for Stock-based Compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award
Percentage of share-based compensation expense			15.00%
Weighted average grant date fair value of options granted during the period (per share)			$ 1.83	$ 9.76
Stock-based compensation	$ 889	$ 1,538	$ 1,905	$ 3,127
Research and development
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	332	549	668	1,127
Research and development \| Employee Stock Purchase Plans
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	12	29	18	51
Research and development \| Equity Incentive Plans
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	320	520	650	1,076
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	557	989	1,237	2,000
General and administrative \| Employee Stock Purchase Plans
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	8	18	14	32
General and administrative \| Equity Incentive Plans
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	$ 549	$ 971	$ 1,223	$ 1,968

Represents the percentage of discount on share price treated as 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.

No definition available.

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- Definition

Amount of noncash expense for share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

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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.

+ References

No definition available.

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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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