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Stock-Based Compensation (Tables) | 3 Months Ended
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Mar. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Allocated Employee Stock-Based Compensation Expense | 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,
| 2018 |  | 2017
Research and development | $ | 3,033 |  |  | $ | 1,478 |
Selling, general and administrative | 6,272 |  |  | 3,235 |
Total stock-based compensation | $ | 9,305 |  |  | $ | 4,713 |
Weighted Average Grant-Date Fair Value of Stock Options and ESPP Purchases | We use the Black-Scholes Merton option pricing model to value our stock options and ESPP purchases. The weighted average grant-date fair value per share of our stock options and ESPP purchases was as follows:
| Three Months Ended March 31,
| 2018 |  | 2017
Stock options | $ | 11.52 |  |  | $ | 9.92 |
ESPP | $ | 7.39 |  |  | $ | 3.71 |
Valuation Assumptions Used to Estimate Grant-Date Fair Value | The grant-date fair value of employee stock option grants and ESPP purchases was estimated using the following assumptions:
| Three Months Ended March 31,
| 2018 |  | 2017
Stock options: |  |  |
Risk-free interest rate | 2.40 | % |  | 1.62 | %
Dividend yield | — | % |  | — | %
Volatility | 54 | % |  | 64 | %
Expected life | 4.0 years |  |  | 4.0 years |
ESPP: |  |  |
Risk-free interest rate | 1.53 | % |  | 0.62 | %
Dividend yield | — | % |  | — | %
Volatility | 53 | % |  | 68 | %
Expected life | 6 months |  |  | 6 months |
Stock Option Activity | Stock option activity for the three months ended March 31, 2018 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, 2017 | 22,208,446 |  |  | $ | 6.83 |  |  |  |  |
Granted | 293,580 |  |  | $ | 25.72 |  |  |  |  |
Exercised | (288,196 | ) |  | $ | 6.69 |  |  |  |  |
Forfeited | (18,484 | ) |  | $ | 12.04 |  |  |  |  |
Options outstanding at March 31, 2018 | 22,195,346 |  |  | $ | 7.08 |  |  | 3.86 years |  | $ | 339,631 |
Exercisable at March 31, 2018 | 16,497,014 |  |  | $ | 4.66 |  |  | 3.30 years |  | $ | 288,535 |
RSU Activity | RSU activity for the three months ended March 31, 2018 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, 2017 | 3,762,990 |  |  | $ | 17.76 |  |  |  |  |
Awarded | 146,790 |  |  | $ | 25.72 |  |  |  |  |
Vested and released | (197,884 | ) |  | $ | 6.30 |  |  |  |  |
Forfeited | (24,494 | ) |  | $ | 17.89 |  |  |  |  |
RSUs outstanding at March 31, 2018 | 3,687,402 |  |  | $ | 18.69 |  |  | 1.83 years |  | $ | 81,676 |

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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 changes in outstanding nonvested restricted stock units. + 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 (c)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -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 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=96867065&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://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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- Definition Tabular disclosure of the weighted-average grant-date fair value of equity options or other equity instruments granted during the year. + 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 (d)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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