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Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2020
Share-based Payment Arrangement [Abstract] |
Summary of Option Activity | A summary of option activity under the Plans for the years ended December 31, 2020, 2019, and 2018 is as follows:
(in thousands, except for share and per share amounts) | Number of Shares |  | Weighted Average Exercise Price at Grant Date |  | Weighted Average Remaining Term (Years) |  | Aggregate Intrinsic Value
Outstanding at December 31, 2017 | 4,719,784 |  |  | $ | 10.03 |  |  | 5.63 |  | $ | 24,421 |
Granted | 567,500 |  |  | 19.22 |  |  |  |  |
Forfeited | (232,527) |  |  | 13.99 |  |  |  |  |
Exercised | (685,715) |  |  | 9.12 |  |  |  |  | 5,945 |
Outstanding at December 31, 2018 | 4,369,042 |  |  | 11.15 |  |  | 5.28 |  | 65,438 |
Granted | 687,500 |  |  | 18.38 |  |  |  |  |
Forfeited | (53) |  |  | 7.94 |  |  |  |  |
Expired | (15,000) |  |  | 14.78 |  |  |  |  |
Exercised | (546,344) |  |  | 11.47 |  |  |  |  | 2,482 |
Outstanding at December 31, 2019 | 4,495,145 |  |  | 12.21 |  |  | 5.58 |  | 22,148 |
Granted | 627,500 |  |  | 11.27 |  |  |  |  |
Forfeited | (225,000) |  |  | 18.83 |  |  |  |  |
Expired | (557,604) |  |  | 14.21 |  |  |  |  |
Exercised | (733,223) |  |  | 7.68 |  |  |  |  | 2,868 |
Outstanding at December 31, 2020 | 3,606,818 |  |  | 12.24 |  |  | 5.76 |  | 8,511 |
Exercisable at December 31, 2020 | 2,556,394 |  |  | 11.52 |  |  | 4.53 |  | 7,302 |
Vested and expected to vest at December 31, 2020 | 3,482,804 |  |  | 12.22 |  |  | 5.64 |  | 8,337 |
Summary of RSU Activity | A summary of RSU activity for the Plans for the years ended December 31, 2020, 2019, and 2018 is as follows:
| Number of Shares |  | Weighted Average Grant Date Fair Value
Unvested at December 31, 2017 | 1,357,838 |  |  | $ | 12.72 |
Granted | 714,086 |  |  | 18.93 |
Forfeited | (229,603) |  |  | 15.19 |
Vested | (528,745) |  |  | 12.69 |
Unvested at December 31, 2018 | 1,313,576 |  |  | 15.68 |
Granted | 937,328 |  |  | 19.46 |
Forfeited | (75,444) |  |  | 18.93 |
Vested | (526,175) |  |  | 14.54 |
Unvested at December 31, 2019 | 1,649,285 |  |  | 18.04 |
Granted | 832,162 |  |  | 11.28 |
Forfeited | (260,127) |  |  | 17.35 |
Vested | (581,757) |  |  | 16.54 |
Unvested at December 31, 2020 | 1,639,563 |  |  | 15.26 |
Stock-Based Compensation Expense | Stock-based compensation expense recognized for the years ended December 31, 2020, 2019 and 2018 comprised of the following:
| Year Ended December 31,
(in thousands) | 2020 |  | 2019 |  | 2018
Research and development | $ | 3,804 |  |  | $ | 3,207 |  |  | $ | 1,290 |
Selling, general and administrative | 9,556 |  |  | 10,251 |  |  | 10,376 |
Total stock-based compensation expense | $ | 13,360 |  |  | $ | 13,458 |  |  | $ | 11,666 |
Black-Scholes-Merton Option Pricing Model for Employee and Director Stock Options Granted | Assumptions used in the Black-Scholes-Merton option pricing model for employee and director stock options granted during the years ended December 31, 2020, 2019 and 2018 were as follows:
| Year Ended December 31,
| 2020 |  | 2019 |  | 2018
Expected dividend yield | — | % |  | — | % |  | — | %
Weighted average expected volatility | 51 | % |  | 58 | % |  | 58 | %
Weighted average expected term (years) | 6.07 |  | 5.95 |  | 5.90
Weighted average risk-free rate | 1.14 | % |  | 2.26 | % |  | 2.68 | %

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- Definition Tabular disclosure of the number and weighted-average grant date fair value for restricted stock units that were outstanding at the beginning and end of the year, and the number of restricted stock units that were granted, vested, or forfeited 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 (c)(1) -URI http://asc.fasb.org/extlink&oid=116856206&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=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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