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Stockholders' Equity | 6 Months Ended
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Jun. 30, 2019
Equity [Abstract] |
Stockholders’ Equity | 9. Stockholders’ Equity Common Stock Offerings In April 2018, we sold 6.9 million shares of our common stock at a public offering price of $26.00 per share. We received total net cash proceeds of $168.7 million (net of $10.7 million in issuance costs) from the sale of the common stock. In June 2018, we sold 5.1 million shares of our common stock at a public offering price of $39.50 per share. We received total net cash proceeds of $194.4 million (net of $5.6 million in issuance costs) from the sale of the common stock. Stock Option Activity The following table summarizes the stock option activity for the three months ended June 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 |  |  | 455 |  |  | $ | 24.47 |  |  |  |  |
Exercised |  |  | (1,542 | ) |  | $ | 10.52 |  |  |  |  |
Expired and forfeited |  |  | (633 | ) |  | $ | 20.83 |  |  |  |  |
Balance at June 30, 2019 |  |  | 13,545 |  |  | $ | 19.31 |  |  |  | 6.99 |
For the six months ended June 30, 2019, 1,542,000 shares of common stock were issued pursuant to the exercise of stock options, resulting in net proceeds of $16.2 million. Stock-based Compensation 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, |
|  | 2019 |  |  | 2018 |  |  | 2019 |  |  | 2018 |
Research and development |  | $ | 4,434 |  |  | $ | 3,115 |  |  | $ | 9,758 |  |  | $ | 6,151 |
General and administrative |  |  | 3,714 |  |  |  | 2,308 |  |  |  | 7,394 |  |  |  | 4,584 |
Sales and marketing |  |  | 4,558 |  |  |  | 2,398 |  |  |  | 13,456 |  |  |  | 4,787 |
Total stock-based compensation expense |  | $ | 12,706 |  |  | $ | 7,821 |  |  | $ | 30,608 |  |  | $ | 15,522 |
As of June 30, 2019, there was $84.5 million of total unrecognized compensation cost related to non-vested, stock-based payment awards granted under all of our equity compensation plans and all non-plan option grants. Total unrecognized compensation cost will be adjusted for future changes in estimated forfeitures. We expect to recognize this compensation cost over a weighted-average period of 2.5 years. 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, |
|  | 2019 |  |  | 2018 |
Risk-free interest rate |  |  | 2.4 | % |  |  | 2.7 | %
Dividend yield |  |  | 0.0 | % |  |  | 0.0 | %
Volatility |  |  | 68.0 | % |  |  | 71.1 | %
Expected life (years) |  |  | 6 |  |  |  | 6 |
We estimated the fair value of each purchase right granted under our 1997 Employee Stock Purchase Plan at the beginning of each new offering period using the Black-Scholes option pricing model with the following weighted-average assumptions:
|  | June 30, |
|  | 2019 |  |  | 2018 |
Risk-free interest rate |  |  | 2.4 | % |  |  | 2.1 | %
Dividend yield |  |  | 0.0 | % |  |  | 0.0 | %
Volatility |  |  | 58.4 | % |  |  | 59.8 | %
Expected life (months) |  |  | 6 |  |  |  | 6 |

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- Definition The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -URI http://asc.fasb.org/topic&trid=2208762 + Details
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