Stock-Based Compensation - Accounting for Stock-based Compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award
Weighted average grant date fair value of options granted during the period (per share)	$ 1.01	$ 1.75	$ 2.51
Stock-based compensation	$ 10,720	$ 6,847	$ 5,442
Research and development
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	6,494	2,719	1,688
Research and development \| Former Executive
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	4,300
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation	$ 4,226	$ 4,128	3,754
General and administrative \| Retiring Director
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation			$ 300

The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

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- Definition

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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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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