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Stockholders' Equity (Tables) | 6 Months Ended
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Jun. 30, 2021
Equity [Abstract] |
Summary of Stock Option Activity | The following table summarizes the stock option activity for the six months ended June 30, 2021:
|  | Shares (in thousands) |  |  | Weighted- average Exercise Price |  |  | Weighted- average Remaining Contractual Term (Years) |
Outstanding at December 31, 2020 |  |  | 18,912 |  |  | $ | 19.59 |  |  |  | 7.21 |
Granted |  |  | 628 |  |  | $ | 15.87 |  |  |  |  |
Exercised |  |  | (358 | ) |  | $ | 15.51 |  |  |  |  |
Expired and forfeited |  |  | (875 | ) |  | $ | 22.50 |  |  |  |  |
Outstanding at June 30, 2021 |  |  | 18,307 |  |  | $ | 19.40 |  |  |  | 7.02 |
Summary of Restricted Stock Unit Activity | The following table summarizes the restricted stock unit activity for the six months ended June 30, 2021:
|  | Shares (in thousands) |  |  | Weighted Average Grant Date Fair Value |
Outstanding at December 31, 2020 |  |  | 603 |  |  | $ | 15.84 |
Granted |  |  | 150 |  |  | $ | 15.29 |
Released |  |  | (77 | ) |  | $ | 15.98 |
Expired and forfeited |  |  | (22 | ) |  | $ | 16.06 |
Outstanding at June 30, 2021 |  |  | 654 |  |  | $ | 15.69 |
Summary of Stock-Based Compensation Expense related to Stock-Based Payment Awards Granted | The following table summarizes stock-based compensation expense related to stock-based payment awards granted pursuant to all of our equity compensation arrangements (in thousands):
|  | Three Months Ended June 30, |  |  | Six Months Ended June 30, |
|  | 2021 |  |  | 2020 |  |  | 2021 |  |  | 2020 |
Research and development |  | $ | 4,672 |  |  | $ | 4,681 |  |  | $ | 9,609 |  |  | $ | 9,968 |
General and administrative |  |  | 2,792 |  |  |  | 3,304 |  |  |  | 5,578 |  |  |  | 6,735 |
Sales and marketing |  |  | 3,777 |  |  |  | 3,129 |  |  |  | 7,540 |  |  |  | 6,385 |
Total stock-based compensation expense |  | $ | 11,241 |  |  | $ | 11,114 |  |  | $ | 22,727 |  |  | $ | 23,088 |
Summary of Fair Value of Option Grant on Grant Date Using Black-Scholes Option Pricing Model | We estimated the fair value of each option grant on the grant date using the Black-Scholes option pricing model with the following weighted-average assumptions:
|  | June 30, |
|  | 2021 |  |  | 2020 |
Risk-free interest rate |  |  | 1.0 | % |  |  | 1.1 | %
Dividend yield |  |  | 0.0 | % |  |  | 0.0 | %
Volatility |  |  | 66.6 | % |  |  | 66.7 | %
Expected life (years) |  |  | 6 |  |  |  | 6 |
Summary of Fair Value of Purchase Right Granted Using Black-Scholes Option Pricing Model | We estimated the fair value of each purchase right granted under our 1997 Employee Stock Purchase Plan, as amended, at the beginning of each new offering period using the Black-Scholes option pricing model with the following weighted-average assumptions:
|  | June 30, |
|  | 2021 |  |  | 2020 |
Risk-free interest rate |  |  | 0.04 | % |  |  | 0.10 | %
Dividend yield |  |  | 0.0 | % |  |  | 0.0 | %
Volatility |  |  | 48.3 | % |  |  | 63.3 | %
Expected life (months) |  |  | 6 |  |  |  | 6 |

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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=120381028&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=120381028&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=120381028&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 employee stock purchase plans, including, but not limited to: (a) expected term, (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=120381028&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=120381028&loc=d3e5070-113901 + Details
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