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Stock-Based Compensation | 6 Months Ended
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Jun. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Stock-Based Compensation | STOCK-BASED COMPENSATION We recorded and allocated employee stock-based compensation expense for our equity incentive plans and our 2000 Employee Stock Purchase Plan (“ESPP”) as follows (in thousands):
| Three Months Ended June 30, |  | Six Months Ended June 30,
| 2017 |  | 2016 |  | 2017 |  | 2016
Research and development expense | $ | 1,600 |  |  | $ | 1,165 |  |  | $ | 3,078 |  |  | $ | 6,729 |
Selling, general and administrative expense | 3,427 |  |  | 2,393 |  |  | 6,662 |  |  | 8,014 |
Total stock-based compensation expense | $ | 5,027 |  |  | $ | 3,558 |  |  | $ | 9,740 |  |  | $ | 14,743 |
We use the Black-Scholes Merton option pricing model to value our stock options and ESPP purchases. The weighted average grant-date fair value of our stock options and ESPP purchases was as follows:
| Three Months Ended June 30, |  | Six Months Ended June 30,
| 2017 |  | 2016 |  | 2017 |  | 2016
Stock options | $ | 10.07 |  |  | $ | 3.17 |  |  | $ | 10.01 |  |  | $ | 2.67 |
ESPP | $ | 5.65 |  |  | $ | 1.60 |  |  | $ | 4.61 |  |  | $ | 1.79 |
The fair value of stock options and ESPP purchases was estimated using the following assumptions:
| Stock Options
| Three Months Ended June 30, |  | Six Months Ended June 30,
| 2017 |  | 2016 |  | 2017 |  | 2016
Risk-free interest rate | 1.72 | % |  | 0.90 | % |  | 1.68 | % |  | 1.10 | %
Dividend yield | — | % |  | — | % |  | — | % |  | — | %
Expected volatility | 60 | % |  | 67 | % |  | 61 | % |  | 76 | %
Expected life | 4.3 years |  |  | 4.4 years |  |  | 4.2 years |  |  | 4.4 years |
| ESPP
| Three Months Ended June 30, |  | Six Months Ended June 30,
| 2017 |  | 2016 |  | 2017 |  | 2016
Risk-free interest rate | 0.94 | % |  | 0.39 | % |  | 0.77 | % |  | 0.42 | %
Dividend yield | — | % |  | — | % |  | — | % |  | — | %
Expected volatility | 60 | % |  | 65 | % |  | 64 | % |  | 69 | %
Expected life | 6 months |  |  | 6 months |  |  | 6 months |  |  | 6 months |
We considered implied volatility as well as our historical volatility in developing our estimate of expected volatility. The expected life computation is based on historical exercise patterns and post-vesting termination behavior. A summary of stock option activity for the six months ended June 30, 2017 is presented below (dollars in thousands, except per share amounts):
| Shares |  | Weighted Average Exercise Price |  | Weighted Average Remaining Contractual Term |  | Aggregate Intrinsic Value
Options outstanding at December 31, 2016 | 24,999,665 |  |  | $ | 4.91 |  |  |  |  |
Granted | 674,110 |  |  | $ | 20.57 |  |  |  |  |
Exercised | (3,242,026 | ) |  | $ | 4.04 |  |  |  |  |
Forfeited | (161,836 | ) |  | $ | 7.27 |  |  |  |  |
Options outstanding at June 30, 2017 | 22,269,913 |  |  | $ | 5.50 |  |  | 4.25 years |  | $ | 426,081 |
Exercisable at June 30, 2017 | 16,181,093 |  |  | $ | 4.13 |  |  | 3.72 years |  | $ | 331,770 |
As of June 30, 2017, a total of 24,237,774 shares were available for grant under our stock option plans. A summary of restricted stock unit (“RSU”) activity for the six months ended June 30, 2017 is presented below (dollars in thousands, except per share amounts):
| Shares |  | Weighted Average Grant Date Fair Value |  | Weighted Average Remaining Contractual Term |  | Aggregate Intrinsic Value
RSUs outstanding at December 31, 2016 | 2,469,791 |  |  | $ | 8.69 |  |  |  |  |
Awarded | 258,272 |  |  | $ | 20.81 |  |  |  |  |
Vested and released | (280,657 | ) |  | $ | 4.24 |  |  |  |  |
Forfeited | (85,543 | ) |  | $ | 10.72 |  |  |  |  |
RSUs outstanding at June 30, 2017 | 2,361,863 |  |  | $ | 10.47 |  |  | 1.72 years |  | $ | 58,173 |

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- Definition The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 40 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=65895303&loc=d3e17540-113929 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=68057994&loc=d3e25284-112666 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) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5444-113901 + Details
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