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Stock-Based Compensation (Tables) | 9 Months Ended
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Sep. 27, 2019
Share-based Payment Arrangement [Abstract] |
Allocated Employee Stock-Based Compensation Expense | We allocated the stock-based compensation expense for our equity incentive plans and our 2000 Employee Stock Purchase Plan (ESPP) as follows (in thousands):
| Three Months Ended September 30, |  | Nine Months Ended September 30,
| 2019 |  | 2018 |  | 2019 |  | 2018
Research and development | $ | 4,301 |  |  | $ | 3,169 |  |  | $ | 13,745 |  |  | $ | 9,102 |
Selling, general and administrative | 8,838 |  |  | 6,573 |  |  | 27,002 |  |  | 19,228 |
Total stock-based compensation | $ | 13,139 |  |  | $ | 9,742 |  |  | $ | 40,747 |  |  | $ | 28,330 |
Weighted Average Grant-Date Fair Value of Stock Options and ESPP Purchases | The weighted average grant-date fair value per share of stock options and ESPP purchases were as follows:
| Three Months Ended September 30, |  | Nine Months Ended September 30,
| 2019 |  | 2018 |  | 2019 |  | 2018
Stock options | $ | 7.71 |  |  | $ | 8.67 |  |  | $ | 8.57 |  |  | $ | 9.13 |
ESPP | $ | 5.39 |  |  | $ | 6.19 |  |  | $ | 4.93 |  |  | $ | 6.96 |
Valuation Assumptions Used to Estimate Grant-Date Fair Value | The grant-date fair value of stock option grants and ESPP purchases was estimated using the following assumptions:
| Three Months Ended September 30, |  | Nine Months Ended September 30,
| 2019 |  | 2018 |  | 2019 |  | 2018
Stock options: |  |  |  |  |  |  |
Risk-free interest rate | 1.57 | % |  | 2.91 | % |  | 1.82 | % |  | 2.83 | %
Dividend yield | — | % |  | — | % |  | — | % |  | — | %
Volatility | 48 | % |  | 55 | % |  | 48 | % |  | 55 | %
Expected life | 4.0 years |  |  | 4.4 years |  |  | 4.4 years |  |  | 4.4 years |
ESPP: |  |  |  |  |  |  |
Risk-free interest rate | 2.09 | % |  | 2.11 | % |  | 2.34 | % |  | 1.74 | %
Dividend yield | — | % |  | — | % |  | — | % |  | — | %
Volatility | 43 | % |  | 51 | % |  | 52 | % |  | 52 | %
Expected life | 6 months |  |  | 6 months |  |  | 6 months |  |  | 6 months |
Stock Option Activity | Activity for stock options during the nine months ended September 30, 2019 was as follows (dollars in thousands, except per share amounts):
| Shares |  | Weighted Average Exercise Price Per Share |  | Weighted Average Remaining Contractual Term |  | Aggregate Intrinsic Value
Options outstanding at December 31, 2018 | 22,674,062 |  |  | $ | 8.71 |  |  |  |  |
Granted | 1,042,286 |  |  | $ | 20.81 |  |  |  |  |
Exercised | (3,042,201 | ) |  | $ | 4.94 |  |  |  |  |
Forfeited | (160,241 | ) |  | $ | 16.76 |  |  |  |  |
Expired | (35,045 | ) |  | $ | 23.41 |  |  |  |  |
Options outstanding at September 30, 2019 | 20,478,861 |  |  | $ | 9.80 |  |  | 3.4 years |  | $ | 186,407 |
Exercisable at September 30, 2019 | 15,860,131 |  |  | $ | 7.02 |  |  | 2.8 years |  | $ | 181,542 |
RSU Activity | Activity for RSUs during the nine months ended September 30, 2019 was as follows (dollars in thousands, except per share amounts):
| Shares |  | Weighted Average Grant Date Fair Value Per Share |  | Weighted Average Remaining Contractual Term |  | Aggregate Intrinsic Value
RSUs outstanding at December 31, 2018 | 4,857,334 |  |  | $ | 18.42 |  |  |  |  |
Awarded | 5,635,362 |  |  | $ | 19.54 |  |  |  |  |
Vested and released | (484,731 | ) |  | $ | 12.33 |  |  |  |  |
Forfeited | (258,142 | ) |  | $ | 18.47 |  |  |  |  |
RSUs outstanding at September 30, 2019 | 9,749,823 |  |  | $ | 19.37 |  |  | 2.2 years |  | $ | 175,448 |

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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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