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Stock-Based Compensation (Tables) | 9 Months Ended
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Sep. 28, 2018
Disclosure of Compensation Related Costs, Share-based Payments [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,
| 2018 |  | 2017 |  | 2018 |  | 2017
Research and development | $ | 3,169 |  |  | $ | 1,663 |  |  | $ | 9,102 |  |  | $ | 4,741 |
Selling, general and administrative | 6,573 |  |  | 3,626 |  |  | 19,228 |  |  | 10,288 |
Total stock-based compensation | $ | 9,742 |  |  | $ | 5,289 |  |  | $ | 28,330 |  |  | $ | 15,029 |
Weighted Average Grant-Date Fair Value of Stock Options and ESPP Purchases | We used a Monte Carlo simulation pricing model to value stock options that include market vesting conditions and a Black-Scholes Merton option pricing model to value other 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,
| 2018 |  | 2017 |  | 2018 |  | 2017
Stock options | $ | 8.67 |  |  | $ | 11.75 |  |  | $ | 9.13 |  |  | $ | 10.32 |
ESPP | $ | 6.19 |  |  | $ | 6.85 |  |  | $ | 6.96 |  |  | $ | 5.29 |
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,
| 2018 |  | 2017 |  | 2018 |  | 2017
Stock options: |  |  |  |  |  |  |
Risk-free interest rate | 2.91 | % |  | 1.70 | % |  | 2.83 | % |  | 1.68 | %
Dividend yield | — | % |  | — | % |  | — | % |  | — | %
Volatility | 55 | % |  | 58 | % |  | 55 | % |  | 61 | %
Expected life | 4.4 years |  |  | 4.0 years |  |  | 4.4 years |  |  | 4.1 years |
ESPP: |  |  |  |  |  |  |
Risk-free interest rate | 2.11 | % |  | 1.14 | % |  | 1.74 | % |  | 0.88 | %
Dividend yield | — | % |  | — | % |  | — | % |  | — | %
Volatility | 51 | % |  | 55 | % |  | 52 | % |  | 61 | %
Expected life | 6 months |  |  | 6 months |  |  | 6 months |  |  | 6 months |
Stock Option Activity | Activity for stock options during the nine months ended September 30, 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 | 3,022,113 |  |  | $ | 19.52 |  |  |  |  |
Exercised | (2,083,457 | ) |  | $ | 5.01 |  |  |  |  |
Forfeited | (216,522 | ) |  | $ | 14.40 |  |  |  |  |
Expired | (5,548 | ) |  | $ | 18.62 |  |  |  |  |
Options outstanding at September 30, 2018 | 22,925,032 |  |  | $ | 8.59 |  |  | 3.9 years |  | $ | 226,821 |
Exercisable at September 30, 2018 | 15,896,370 |  |  | $ | 5.03 |  |  | 3.1 years |  | $ | 203,118 |
RSU Activity | Activity for RSUs during the nine months ended September 30, 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 | 2,411,245 |  |  | $ | 18.50 |  |  |  |  |
Vested and released | (374,992 | ) |  | $ | 8.18 |  |  |  |  |
Forfeited | (249,362 | ) |  | $ | 18.64 |  |  |  |  |
RSUs outstanding at September 30, 2018 | 5,549,881 |  |  | $ | 18.69 |  |  | 2.1 years |  | $ | 98,344 |

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -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=115993241&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5047-113901 + Details
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- Definition Tabular disclosure of the changes in outstanding nonvested restricted stock units. + 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 (c)(2) -URI http://asc.fasb.org/extlink&oid=109197908&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://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=109197908&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=109197908&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=109197908&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=109197908&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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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