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Stock- based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ / shares in Units, $ in Thousands | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | 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) | 4,146,064 | 3,572,116 |  |  |  |
Weighted average exercise price (in dollars per share) | $ 16.90 | $ 11.40 |  |  |  |
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |  |  |  |
Stock-based compensation expense | $ 7,847 | $ 3,244 | $ 862 |  |  |
Stock Option Plan 2000 [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Share-based compensation, number of shares authorized (in shares) | 150,297 |  |  |  |  |
Share-based compensation arrangement by share-based payment award, options, outstanding, number (in shares) | 19,571 |  |  |  |  |
Weighted average exercise price (in dollars per share) | $ 0.81 |  |  |  |  |
Equity Incentive Plan 2003 [Member] |  |  |  |  |  |
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) | 1,649,193 |  |  |  |  |
Weighted average exercise price (in dollars per share) | $ 1.72 |  |  |  |  |
Stock Incentive Plan 2013 [Member] |  |  |  |  |  |
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) | 4,087,098 |  |  |  |  |
Share-based compensation arrangement by share-based payment award, options, outstanding, number (in shares) | 2,477,300 |  |  |  |  |
Weighted average exercise price (in dollars per share) | $ 27.13 |  |  |  |  |
Stock reserved for future issuance (in shares) | 1,960,168 |  |  |  |  |
Percentage of common stock share outstanding | 4.00% |  |  |  |  |
Research and Development [Member] |  |  |  |  |  |
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |  |  |  |
Stock-based compensation expense | $ 3,623 | 1,562 | 507 |  |  |
General and Administrative [Member] |  |  |  |  |  |
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |  |  |  |
Stock-based compensation expense | $ 4,224 | $ 1,682 | $ 355 |  |  |

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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=6415400&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=6415400&loc=d3e5070-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F + Details
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