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Stockholders' Equity (Tables) | 9 Months Ended
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Sep. 30, 2019
Equity [Abstract] |
Summary of Stock Option Activity | The following table summarizes the stock option activity for the three months ended September 30, 2019:
|  | Shares (in thousands) |  |  | Weighted- average Exercise Price |  |  | Weighted- average Remaining Contractual Term (Years) |
Balance at December 31, 2018 |  |  | 15,265 |  |  | $ | 18.33 |  |  |  | 6.87 |
Granted |  |  | 985 |  |  | $ | 21.18 |  |  |  |  |
Exercised |  |  | (1,793 | ) |  | $ | 11.29 |  |  |  |  |
Expired and forfeited |  |  | (1,378 | ) |  | $ | 22.65 |  |  |  |  |
Balance at September 30, 2019 |  |  | 13,079 |  |  | $ | 19.06 |  |  |  | 7.34 |
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 September 30, |  |  | Nine Months Ended September 30, |
|  | 2019 |  |  | 2018 |  |  | 2019 |  |  | 2018 |
Research and development |  | $ | 4,365 |  |  | $ | 3,212 |  |  | $ | 14,123 |  |  | $ | 9,363 |
General and administrative |  |  | 2,939 |  |  |  | 2,366 |  |  |  | 10,333 |  |  |  | 6,950 |
Sales and marketing |  |  | 2,400 |  |  |  | 2,472 |  |  |  | 15,856 |  |  |  | 7,259 |
Total stock-based compensation expense |  | $ | 9,704 |  |  | $ | 8,050 |  |  | $ | 40,312 |  |  | $ | 23,572 |
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:
|  | September 30, |
|  | 2019 |  |  | 2018 |
Risk-free interest rate |  |  | 2.1 | % |  |  | 2.8 | %
Dividend yield |  |  | 0.0 | % |  |  | 0.0 | %
Volatility |  |  | 67.4 | % |  |  | 70.5 | %
Expected life (years) |  |  | 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=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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