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Stock-Based Compensation | 3 Months Ended
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Mar. 31, 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 March 31,
| 2017 |  | 2016
Research and development expense | $ | 1,478 |  |  | $ | 5,564 |
Selling, general and administrative expense | 3,235 |  |  | 5,621 |
Total stock-based compensation expense | $ | 4,713 |  |  | $ | 11,185 |
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 March 31,
| 2017 |  | 2016
Stock options | $ | 9.92 |  |  | $ | 2.51 |
ESPP | $ | 3.71 |  |  | $ | 2.31 |
The fair value of stock options and ESPP purchases was estimated using the following assumptions:
| Stock Options
| Three Months Ended March 31,
| 2017 |  | 2016
Risk-free interest rate | 1.62 | % |  | 1.16 | %
Dividend yield | — | % |  | — | %
Expected volatility | 64 | % |  | 79 | %
Expected life | 4.0 years |  |  | 4.3 years |
| Employee Stock Purchase Plan
| Three Months Ended March 31,
| 2017 |  | 2016
Risk-free interest rate | 0.62 | % |  | 0.51 | %
Dividend yield | — | % |  | — | %
Expected volatility | 68 | % |  | 81 | %
Expected life | 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 three months ended March 31, 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 | 288,120 |  |  | $ | 20.16 |  |  |  |  |
Exercised | (2,297,358 | ) |  | $ | 4.29 |  |  |  |  |
Forfeited | (141,021 | ) |  | $ | 7.23 |  |  |  |  |
Options outstanding at March 31, 2017 | 22,849,406 |  |  | $ | 5.15 |  |  | 4.46 years |  | $ | 377,392 |
Exercisable at March 31, 2017 | 16,456,810 |  |  | $ | 4.02 |  |  | 3.94 years |  | $ | 290,400 |
As of March 31, 2017, a total of 1,548,149 shares were available for grant under our stock option plans A summary of restricted stock unit (“RSU”) activity for the three months ended March 31, 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
Awards outstanding at December 31, 2016 | 2,469,791 |  |  | $ | 8.69 |  |  |  |  |
Awarded | 135,750 |  |  | $ | 20.06 |  |  |  |  |
Vested and released | (178,898 | ) |  | $ | 4.42 |  |  |  |  |
Forfeited | (73,495 | ) |  | $ | 10.36 |  |  |  |  |
Awards outstanding at March 31, 2017 | 2,353,148 |  |  | $ | 9.62 |  |  | 1.87 years |  | $ | 50,993 |

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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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