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Stock-based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [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,
| 2017 |  | 2016 |  | 2015
Research and development | $ | 7,388 |  |  | $ | 5,778 |  |  | $ | 3,623 |
General and administrative | 7,356 |  |  | 6,387 |  |  | 4,224 |
Total stock-based compensation expense | $ | 14,744 |  |  | $ | 12,165 |  |  | $ | 7,847 |
Schedule of Employee Stock Options Award 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,
| 2017 |  | 2016 |  | 2015
Expected dividend yield | 0% |  | 0% |  | 0%
Expected volatility | 67% - 68% |  | 64% - 69% |  | 73% - 75%
Risk-free interest rate | 1.9% - 2.3% |  | 1.2% - 2.4% |  | 1.6% - 2.1%
Expected term | 6.25 years |  | 6.25 years |  | 6.25 years
Schedule of Stock Option and Restricted Stock Unit Activity | The following table summarizes stock option and restricted stock unit (RSU) activity for 2017:
| Shares |  | Weighted-Average Exercise Price |  | Weighted-Average Remaining Contractual Term (Years) |  | Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2016 | 3,838,060 |  |  | $ | 18.93 |  |  | 7.0 |  |
Granted | 1,246,225 |  |  | 20.05 |  |  |  |  |
Options exercised or RSUs vested | (253,036 | ) |  | 2.16 |  |  |  |  |
Forfeited or expired | (326,607 | ) |  | 24.26 |  |  |  |  |
Outstanding, December 31, 2017 | 4,504,642 |  |  | 19.79 |  |  | 7.0 |  | $ | 18,343 |
December 31, 2017: |  |  |  |  |  |  |  |  |  |
Exercisable | 2,910,171 |  |  | 17.65 |  |  | 6.1 |  | 17,495 |
Vested and expected to vest | 4,337,015 |  |  | 19.60 |  |  | 6.9 |  | 18,275 |

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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=96867065&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=96867065&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 + 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=96867065&loc=d3e5070-113901 + Details
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