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.


	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.

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:

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.


	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

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.

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.


	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.


	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.

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.

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.

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.

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.