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Stock-based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2015
Stock-based Compensation [Abstract] |
Schedule of Stock-Based Compensation Expense | The following stock-based compensation amounts were recognized for the periods indicated (in thousands):
| Year Ended December 31, |
| 2015 |  | 2014 |  | 2013 |
Research and development |  | $ | 3,623 |  |  | $ | 1,562 |  |  | $ | 507 |
General and administrative |  |  | 4,224 |  |  |  | 1,682 |  |  |  | 355 |
Total stock-based compensation expense |  | $ | 7,847 |  |  | $ | 3,244 |  |  | $ | 862 |
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions | The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions in the following table:
| Year Ended December 31,
| 2015 |  | 2014 |  | 2013
Expected dividend yield |  | 0% |  |  | 0% |  |  | 0%
Expected volatility |  | 73% - 75% |  |  | 67% |  |  | 53% - 67%
Risk-free interest rate |  | 1.6% - 2.1% |  |  | 1.8% - 2.3% |  |  | 1.2% - 2.2%
Expected term | 6.25 years |  | 6.25 years |  | 7 years
Schedule of Stock Option and Restricted Stock Unit Activity | The following table summarizes stock option and restricted stock unit (RSU) activity for 2015:
|  | Shares |  |  | Weighted-Average Exercise Price |  |  | Weighted-Average Remaining Contractual Term (Years) |  | Aggregate Intrinsic Value (in thousands) |
Outstanding, December 31, 2014 |  |  | 3,572,116 |  |  | $ | 11.40 |  |  |  | 7.3 |  |  |
Granted |  |  | 987,680 |  |  |  | 31.59 |  |  |  |  |  |  |
Exercised |  |  | (374,214 | ) |  |  | 2.45 |  |  |  |  |  |  |
Forfeited or expired |  |  | (39,518 | ) |  |  | 23.44 |  |  |  |  |  |  |
Outstanding, December 31, 2015 |  |  | 4,146,064 |  |  |  | 16.90 |  |  |  | 7.4 |  |  | $ | 59,344 |
December 31, 2015: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exercisable |  |  | 1,992,951 |  |  |  | 7.94 |  |  |  | 5.7 |  |  |  | 46,012 |
Vested and expected to vest |  |  | 3,894,148 |  |  |  | 16.50 |  |  |  | 7.3 |  |  |  | 57,267 |

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- Definition Tabular disclosure of the allocation of equity-based compensation costs to a given line item on the balance sheet and income statement for the period. This may include the reporting line for the costs and the amount capitalized and expensed. + 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 (h)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 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 S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 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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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + 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 (f)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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