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Stockholders' Equity | 12 Months Ended
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Dec. 31, 2020
Share-based Payment Arrangement [Abstract] |
Stockholders' Equity | Stockholders' Equity Securities Repurchase Program In June 2020, our board of directors approved a securities repurchase program authorizing our repurchase of up to $500.0 million of our common stock and/or convertible notes, through open market purchases, block trades, and/or privately negotiated transactions or pursuant to Rule 10b5-1 plans, in compliance with applicable securities laws and other legal requirements. The timing, volume, and nature of the repurchases will be determined by management based on the capital needs of the business, market conditions, applicable legal requirements, and other factors. During the year ended December 31, 2020, we repurchased $57.4 million of aggregate principal amount of the 2023 notes in privately-negotiated transactions for an aggregate consideration of $149.6 million and have $350.4 million remaining under the repurchase program. The repurchase program will end on December 31, 2021. Share-based Compensation Expense Total share-based compensation expense recorded for employees and non-employees, is as follows (in thousands):
|  |  | Years Ended December 31,
|  |  |  |  | 2020 |  | 2019 |  | 2018
Cost of revenues |  |  |  |  | $ | 950 |  |  | $ | 426 |  |  | $ | 420 |
Research and development |  |  |  |  | 31,588 |  |  | 22,229 |  |  | 17,055 |
Sales and marketing |  |  |  |  | 9,606 |  |  | 7,380 |  |  | 6,703 |
General and administrative |  |  |  |  | 41,911 |  |  | 34,874 |  |  | 27,852 |
Total share-based compensation expense |  |  |  |  | $ | 84,055 |  |  | $ | 64,909 |  |  | $ | 52,030 |
During the year ended December 31, 2020, the Compensation Committee of the Board of Directors approved a modification accelerating the vesting of 42,982 restricted stock units (RSUs) and performance-based restricted stock units (PSUs) for three key employees resulting in approximately $2.4 million of incremental share-based compensation expense. As of December 31, 2020, we had a total of approximately $110.3 million of unrecognized share-based compensation expense related to RSUs and PSUs that is expected to be recognized over the remaining weighted average period of 1.8 years. Fair Value of 2013 ESPP Under the 2013 ESPP, rights to purchase shares are generally granted during the second and fourth quarter of each year. We estimate the fair value of each right to purchase shares under our 2013 ESPP using the Black-Scholes-Merton option-pricing model, which utilizes the fair value of our common stock based on active market and requires input on the following subjective assumptions: Expected Term. The expected term for rights to purchase shares under the 2013 ESPP is six months. Expected Volatility. The expected volatility is based on the average volatility of our stock price over the expected term. Expected Dividends. The dividend assumption is based on our historical experience. To date we have not paid any dividends on our common stock. Risk-Free Interest Rate. The risk-free interest rate used in the valuation method is the implied yield currently available on the United States treasury zero-coupon issues, with a remaining term equal to the expected term. The following table summarizes the key assumptions used to determine the fair value of rights granted under the 2013 ESPP:
|  |  | Years Ended December 31,
|  |  |  |  | 2020 |  | 2019 |  | 2018
Expected term (years) |  |  |  |  | 0.50 |  | 0.50 |  | 0.50
Expected volatility |  |  |  |  | 52.06%-68.09% |  | 40.51%-41.81% |  | 42.07%-44.97%
Dividend yield |  |  |  |  | — | % |  | — | % |  | — | %
Risk-free interest rate |  |  |  |  | 0.12%-0.15% |  | 1.59%-2.43% |  | 2.09%-2.50%
Weighted-average grant-date fair value per share |  |  |  |  | $ | 20.52 |  |  | $ | 9.88 |  |  | $ | 7.14 |
2013 ESPP Activity There were 173,992 shares purchased under the 2013 ESPP for the year ended December 31, 2020 at an average price per share of $38.85 with cash proceeds from the issuance of shares of $6.8 million. There were 201,581 shares purchased under the 2013 ESPP for the year ended December 31, 2019 at an average price per share of $25.55 with cash proceeds from the issuance of shares of $5.1 million. There were 253,301 shares purchased under the 2013 ESPP for the year ended December 31, 2018 at an average price per share of $15.77 with cash proceeds from the issuance of shares of $4.0 million. Fair Value of RSUs and PSUs RSUs and PSUs are converted into shares of our common stock upon vesting on a one-for-one basis. Vesting of RSUs is subject to the employee’s continuing service to us, while vesting of PSUs is subject to our achievement of specified corporate financial performance objectives in addition to the employee's continuing service to us. RSUs are typically fully vested at the end of three or four years while PSUs vest subject to the achievement of performance objectives and if achieved, typically vest over two to three years. We assess the achievement of performance objectives on a quarterly basis and adjust our share-based compensation expense as appropriate. Stock Option Activity
|  |  | Options Outstanding
|  |  | Number of Options Outstanding |  | Weighted- Average Exercise Price per Share |  | Weighted-Average Remaining Contractual Term in Years |  | Aggregate Intrinsic Value
Balance at December 31, 2019 |  |  | 1,611,385 |  |  | $ | 8.64 |  |  | 3.60 |  | $ | 47,171,160 |
Exercised |  |  | (984,068) |  |  | 9.13 |  |  |  |  |
Balance at December 31, 2020 |  |  | 627,317 |  |  | $ | 7.86 |  |  | 3.48 |  | $ | 51,733,285 |
We did not grant any stock option awards during the years ended December 31, 2020, 2019, and 2018. The total intrinsic value of options exercised during the years ended December 31, 2020, 2019 and 2018, was approximately $53.5 million, $90.8 million, and $57.2 million, respectively. RSU and PSU Activity
| RSUs and PSUs Outstanding
| Number of RSUs and PSUs Outstanding |  | Weighted Average Grant Date Fair Value
Balance at December 31, 2019 | 6,909,530 |  |  | $ | 24.04 |
Granted | 2,593,745 |  |  | 45.37 |
Released | (4,098,742) |  |  | 20.22 |
Canceled | (588,533) |  |  | 31.84 |
Balance at December 31, 2020 | 4,816,000 |  |  | $ | 37.82 |
The weighted-average grant-date fair value of RSUs and PSUs granted during the years ended December 31, 2020, 2019, and 2018 was $45.37, $37.56, and $21.67, respectively. The total fair value of RSUs and PSUs vested as of the vesting dates during the years ended December 31, 2020, 2019, and 2018 was $200.1 million, $222.3 million, and $120.9 million, respectively. 2020 PSU Grants In March 2020, we granted PSUs under the 2013 Plan to certain of our key executives. The PSUs entitle the executives to receive a certain number of shares of our common stock based on our satisfaction of certain financial and strategic performance targets during 2020. Based on the achievement of the performance conditions for the March 2020 grants, the final settlement met the target threshold based on a specified objective formula approved by the Compensation Committee. These PSUs will vest over a three-year period, with the initial vesting occurring in March 2021. The number of shares underlying the March 2020 PSUs granted during the year ended December 31, 2020 totaled 460,976 shares and had a grant date fair value of $39.21 per share. 2019 PSU Grants In March 2019, we granted PSUs under the 2013 Plan to certain of our key executives. The PSUs entitle the executives to receive a certain number of shares of our common stock based on our satisfaction of certain financial and strategic performance targets during 2019. Based on the achievement of the performance conditions for the March 2019 grants, the final settlement met the target threshold based on a specified objective formula approved by the Compensation Committee. These PSUs will vest over a three-year period, with the initial vesting occurring in March 2020. The number of shares underlying the March 2019 PSUs granted during the year ended December 31, 2019 totaled 436,042 shares and had a grant date fair value of $40.42 per share. 2018 PSU Grants In August 2018, in conjunction with our acquisition of StudyBlue, we granted PSUs under the 2013 Plan to certain employees. The PSUs entitle the employees to receive a certain number of shares of our common stock based on our satisfaction of certain strategic performance targets during 2018 and 2019. Based on the achievement of the performance conditions for the August 2018 grant, the final settlement exceeded the target threshold based on a specified objective formula approved by the Compensation Committee. These PSUs vest over a three-year period, with the initial vesting occurring in September 2019. The number of shares underlying the August 2018 PSUs granted during the year ended December 31, 2018 totaled 45,756 shares and had a grant date fair value of $28.74 per share. In March 2018, we granted PSUs under the 2013 Plan to certain of our key executives. The PSUs entitle the executives to receive a certain number of shares of our common stock based on our satisfaction of certain financial and strategic performance targets during 2018. Based on the achievement of the performance conditions for the March 2018 grant, the final settlement exceeded the target threshold based on a specified objective formula approved by the Compensation Committee. These PSUs vest over a three-year period, with the initial vesting occurring in March 2019. The number of shares underlying the March 2018 PSUs granted during the year ended December 31, 2018 totaled 845,934 shares and had a grant date fair value of $19.70 per share.

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