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Stock- based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ / shares in Units, $ in Thousands | 12 Months Ended | 84 Months Ended |  |
--- | --- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2012 | Oct. 31, 2013 | Feb. 28, 2003
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award, options, outstanding, number (in shares) | 4,504,642 | 3,838,060 |  |  |  |
Weighted average exercise price (in dollars per share) | $ 19.79 | $ 18.93 |  |  |  |
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |  |  |  |
Stock-based compensation expense | $ 14,744 | $ 12,165 | $ 7,847 |  |  |
Equity Incentive Plan 2003 |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Share-based compensation, number of shares authorized (in shares) |  |  |  |  |  | 2,051,644
Increased number of shares authorized (in shares) |  |  |  | 4,336,730 |  |
Share-based compensation arrangement by share-based payment award, options, outstanding, number (in shares) | 955,296 |  |  |  |  |
Weighted average exercise price (in dollars per share) | $ 1.85 |  |  |  |  |
Stock Incentive Plan 2013 |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Share-based compensation, number of shares authorized (in shares) |  |  |  |  | 1,960,168 |
Increased number of shares authorized (in shares) | 6,769,888 |  |  |  |  |
Share-based compensation arrangement by share-based payment award, options, outstanding, number (in shares) | 3,549,346 |  |  |  |  |
Weighted average exercise price (in dollars per share) | $ 24.62 |  |  |  |  |
Stock reserved for future issuance (in shares) | 1,960,168 |  |  |  |  |
Percentage of common stock share outstanding | 4.00% |  |  |  |  |
Research and development |  |  |  |  |  |
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |  |  |  |
Stock-based compensation expense | $ 7,388 | 5,778 | 3,623 |  |  |
General and administrative |  |  |  |  |  |
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |  |  |  |
Stock-based compensation expense | $ 7,356 | $ 6,387 | $ 4,224 |  |  |

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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5047-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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