The Company’s stock-based compensation program is a long-term retention program that is intended to attract and reward talented employees and align stockholder and employee interests. As of December 31, 2019, the Company had one stock-based compensation plan under which it was granting equity awards. The Company is currently granting stock-based awards from its Amended and Restated 2014 Equity Incentive Plan (the "2014 Plan"), which was approved at the Company's Annual Meeting of Stockholders on June 22, 2017. In March 2019, the Company's Board of Directors adopted an amendment to the 2014 Plan, which was approved at the Company's Annual Meeting of Stockholders on June 4, 2019. The Company’s superseded stock plans with outstanding awards include the Amended and Restated 2005 Equity Incentive Plan.

and to make stock-based awards to full and part-time employees of the Company and its subsidiaries or affiliates, where legally eligible to participate, as well as to consultants and non-employee directors of the Company. ISOs, NSOs, and SARs are not currently being granted. Pursuant to the June 2019 amendment, the maximum number of shares of common stock available for issuance under the 2014 Plan was reduced to 43,400,000. In addition, the amendment removes the fungible share adjustment used to determine shares available for issuance. Under the original terms of the 2014 Plan, shares available for issuance were adjusted by a 2.75 fungible share factor. Pursuant to the amendment, beginning on June 4, 2019, each share award granted under the 2014 Plan will reduce the share reserve by one share and all share awards granted on June 4, 2019 and thereafter that are later forfeited, canceled or terminated will be returned to the share reserve in the same manner. Under the 2014 Plan, NSOs must be granted at exercise prices no less than fair market value on the date of grant. Non-vested stock awards may be granted for such consideration in cash, other property or services, or a combination thereof, as determined by the Company’s Compensation Committee of its Board of Directors. Stock-based awards are generally exercisable or issuable upon vesting. The Company’s policy is to recognize compensation cost for awards with only service conditions and a graded vesting schedule on a straight-line basis over the requisite service period for the entire award. As of December 31, 2019, there were 11,894,175 shares of common stock reserved for issuance pursuant to the Company’s stock-based compensation plans including authorization under its 2014 Plan to grant stock-based awards covering 6,156,746 shares of common stock.

In December 2014, the Company’s Board of Directors approved the 2015 Employee Stock Purchase Plan (the “2015 ESPP”), which was approved by stockholders at the Company’s Annual Meeting of Stockholders held on May 28, 2015. Under the 2015 ESPP, all full-time and certain part-time employees of the Company are eligible to purchase common stock of the Company twice per year at the end of a six-month payment period (a “Payment Period”). During each Payment Period, eligible employees who so elect may authorize payroll deductions in an amount no less than 1% nor greater than 10% of his or her base pay for each payroll period in the Payment Period. At the end of each Payment Period, the accumulated deductions are used to purchase shares of common stock from the Company up to a maximum of 12,000 shares for any one employee during a Payment Period. Shares are purchased at a price equal to 85% of the fair market value of the Company’s common stock, on either the first business day of the Payment Period or the last business day of the Payment Period, whichever is lower. Employees who, after exercising their rights to purchase shares of common stock in the 2015 ESPP, would own shares representing 5% or more of the voting power of the Company’s common stock, are ineligible to continue to participate under the 2015 ESPP. The 2015 ESPP provides for the issuance of a maximum of 16,000,000 shares of common stock. As of December 31, 2019, 2,192,755 shares have been issued under the 2015 ESPP. The Company recorded stock-based compensation costs related to its employee stock purchase plan of $12.4 million, $9.8 million and $10.0 million for the years ended December 31, 2019, 2018 and 2017, respectively.

The Company determined the expected volatility factor by considering the implied volatility in six-month market-traded options of the Company's common stock based on third party volatility quotes. The Company's decision to use implied volatility was based upon the availability of actively traded options on the Company's common stock and its assessment that implied volatility is more representative of future stock price trends than historical volatility. The risk-free interest rate was based on a U.S. Treasury instrument whose term is consistent with the expected term of the stock options. The Company's historical dividend yield input was zero in prior periods as it has not historically paid cash dividends on its common stock. The current dividend yield has been updated for expected dividend yield payout given the Company started paying a recurring quarterly dividend beginning in December 2018. The expected term is based on the term of the purchase period for grants made under the ESPP.

In March 2017, the Company granted senior level employees non-vested stock unit awards representing, in the aggregate, 275,148 non-vested stock units that vest based on certain target performance and service conditions. The number of non-vested stock units underlying the award was determined within sixty days of the three-year performance period ending December 31, 2019. The attainment level under the award will be based on the Company's relative total return to stockholders over the performance period compared to a pre-established custom index group. If the Company’s relative total return to stockholders is between the 41st percentile and the 80th percentile when compared to the index companies, the number of non-vested stock units earned will be based on interpolation. The maximum number of non-vested stock units that may vest pursuant to the awards is capped at 200% of the target number of non-vested stock units set forth in the award agreement and is earned if the Company's relative total return to stockholders when compared to the index companies is at or greater than the 80th percentile. If the Company’s total return to stockholders is negative, the number of non-vested stock units earned will be no more than 100% regardless of the Company’s relative total return to stockholders compared to the index companies. If the awardee is not employed by the Company at the end of the performance period, the extent to which the awardee will vest in the award, if at all, is dependent upon the timing and character of the termination as provided in the award agreement. Each non-vested stock unit, upon vesting, represents the right to receive one share of the Company's common stock. In December 2018, certain awards for senior level employees, none of whom were executive officers, were modified to replace the pre-established custom index group used to measure performance and related award payout to companies that are part of the Nasdaq Composite index. As a result, the awards were revalued as of the modification date. The impact of the modification was not material to the consolidated financial statements.

The market condition requirements are reflected in the grant date fair value of the award, and the compensation expense for the award will be recognized assuming that the requisite service is rendered regardless of whether the market conditions are achieved. The grant date fair value of the non-vested performance stock unit awards was determined through the use of a Monte Carlo simulation model, which utilized multiple input variables that determined the probability of satisfying the market condition requirements applicable to each award as follows:

For the unmodified March 2017 grant, the range of expected volatilities utilized was based on the historical volatilities of the Company's common stock and the average of its peer group. The Company chose to use historical volatility to value these awards because historical stock prices were used to develop the correlation coefficients between the Company and its peer group in order to model the stock price movements. The volatilities used were calculated over the most recent 2.75 year period,

For the modified March 2017 grant, all input variables chosen are as of the modification date. The range of expected volatilities utilized was based on the historical volatilities of the Company's common stock and the average of the Nasdaq Composite index peer group. The Company chose to use historical volatility to value these awards because historical stock prices were used to develop the correlation coefficients between the Company and its peer group in order to model the stock price movements. The volatilities used were calculated over the most recent 1.06 year period, which is commensurate with the awards' remaining performance period at the modification date. The risk free interest rate was based on the implied yield available on U.S. Treasury zero-coupon issues with remaining terms equivalent to the remaining performance period. The Company used a zero dividend yield input for this award as dividends are assumed to be reinvested. The estimated incremental fair value of each modified award as of the modification date was $99.54.

The Company also awards senior level employees, certain other employees and new non-employee directors, non-vested stock units granted under the 2014 Plan that vest based on service. The majority of these non-vested stock unit awards generally vest 33.33% on each anniversary subsequent to the date of the award. Each non-vested stock unit, upon vesting, represents the right to receive one share of the Company’s common stock. In addition, the Company awards non-vested stock units to all of its continuing non-employee directors. These awards vest monthly in 12 equal installments based on service and, upon vesting, each stock unit represents the right to receive one share of the Company's common stock.

In April 2019, the Company awarded senior level employees 293,991 non-vested performance stock unit awards granted under the 2014 Plan. The number of non-vested stock units underlying the award will be determined within sixty days following completion of the performance period ending December 31, 2021 and will be based on the achievement of specific corporate financial performance goals related to subscription bookings as a percentage of total subscription and product bookings measured during the period from January 1, 2021 to December 31, 2021. The number of non-vested stock units issued will be based on a graduated slope, with the maximum number of non-vested stock units issuable pursuant to the award capped at 200% of the target number of non-vested stock units set forth in the award agreement. The Company is required to estimate the attainment expected to be achieved related to the defined performance goals and the number of non-vested stock units that will ultimately be awarded in order to recognize compensation expense over the vesting period. Each non-vested stock unit, upon vesting, represents the right to receive one share of the Company’s common stock. Compensation expense will be recorded through the end of the performance period on December 31, 2021 if it is deemed probable that the performance goals will be met. If the performance goals are not met, no compensation cost will be recognized and any previously recognized compensation cost will be reversed.

In February 2019, the Company had awarded certain senior level employees 93,500 non-vested performance stock units granted under the 2014 Plan. The number of non-vested stock units underlying the award were to be determined within sixty days following the completion of the performance period ending December 31, 2020 and were based on the achievement of specific corporate financial performance goals between the fiscal years ended December 31, 2018 and December 31, 2020. The Company was required to estimate the attainment expected to be achieved related to the defined performance goals and the number of non-vested stock units that would have ultimately been awarded in order to recognize compensation expense over the vesting period. Each non-vested stock unit, upon vesting, represented the right to receive one share of the Company’s common stock. Compensation expense would have been recorded through the end of the performance period on December 31, 2020 if it was deemed probable that the performance goals would have been met. In January 2020, the non-vested performance stock units were cancelled pursuant to a forfeiture agreement executed by each holder in return for nominal cash consideration. The impact of the cancellation was not material to the consolidated financial statements.

In March 2018, the Company awarded senior level employees 268,729 non-vested performance stock unit awards granted under the 2014 Plan. The number of non-vested stock units underlying the award will be determined within sixty days following completion of the performance period ending December 31, 2020 and will be based on the achievement of specific corporate financial performance goals related to subscription bookings as a percentage of total product bookings measured

during the period from January 1, 2020 to December 31, 2020. As defined in the applicable award agreements, total product bookings includes subscription bookings. The number of non-vested stock units issued will be based on a graduated slope, with the maximum number of non-vested stock units issuable pursuant to the award capped at 200% of the target number of non-vested stock units set forth in the award agreement. The Company is required to estimate the attainment expected to be achieved related to the defined performance goals and the number of non-vested stock units that will ultimately be awarded in order to recognize compensation expense over the vesting period. Each non-vested stock unit, upon vesting, represents the right to receive one share of the Company’s common stock. Compensation expense will be recorded through the end of the performance period on December 31, 2020 if it is deemed probable that the performance goals will be met. If the performance goals are not met, no compensation cost will be recognized and any previously recognized compensation cost will be reversed.

On August 1, 2017, the Company awarded certain senior level employees 184,322 non-vested performance stock unit granted under the 2014 Plan. The number of non-vested stock units underlying each award was determined within sixty days of the calendar year following completion of the performance period ending December 31, 2019 and was based on achievement of specific corporate financial performance goals related to non-GAAP net operating margin and subscription bookings as a percent of total product bookings measured during the period from January 1, 2019 to December 31, 2019. As defined in the applicable award agreements, total product bookings includes subscription bookings. The number of non-vested stock units issued will be based on a graduated slope, with the maximum number of non-vested stock units issuable pursuant to the award capped at 200% of the target number of non-vested stock units set forth in the award agreement. The Company is required to estimate the attainment expected to be achieved related to the defined performance goals and the number of non-vested stock units that will ultimately be awarded in order to recognize compensation expense over the vesting period. Each non-vested stock unit, upon vesting, represents the right to receive one share of the Company’s common stock. The non-GAAP net operating margin and subscription bookings as a percent of total product targets were set in the first quarter of 2018. As a result, such awards were not outstanding under U.S. GAAP until the first quarter of 2018 when the performance goals were determined and subsequently communicated to employees who received these awards. Finally, these awards vested on December 31, 2019 and met the underlying performance metrics. As a result, compensation expense was recorded through the end of the performance period. The final payout approval related to the awards was obtained within sixty days of the vesting date in accordance with the award provisions.

On April 22, 2019, the change in control provisions of the unvested and outstanding March 2017 market performance stock unit awards and the February 2019, March 2018 and August 2017 company performance stock unit awards were modified such that if a change in control were to occur prior to the end of the award’s performance period, the award would be deemed earned at 200% of the target award, subject to time-based vesting and the awardee’s continuous employment through the end of the award’s performance periods. Previously, the change in control provisions of these awards allowed for either pro rata vesting or vesting based on interim performance through the change in control date. No incremental compensation expense was recorded as a result of this modification given the improbable nature of a change in control event.

For the years ended December 31, 2019, 2018 and 2017, the Company recognized stock-based compensation expense of $266.5 million, $193.8 million and $149.8 million, respectively, related to non-vested stock units. The fair value of the non-vested stock units released in 2019, 2018, and 2017 was $246.7 million, $149.3 million and $150.0 million, respectively. As of December 31, 2019, there was $392.1 million of total unrecognized compensation cost related to non-vested stock units. The unrecognized cost is expected to be recognized over a weighted-average period of 1.69 years.