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Equity Incentive Plans and Stock-Based Compensation
 3 Months Ended
Mar. 31, 2024
Share-Based Payment Arrangement [Abstract]  
Equity Incentive Plans and Stock-Based Compensation Equity Incentive Plans and Stock-Based Compensation
A summary of shares available for grant under the Company’s plans is as follows:
 
Shares Available
 for Grant
Total shares available for grant as of December 31, 202311,954,150
Stock options expired1,125
Nonvested equity stock and stock units granted (1)
(54,262)
Nonvested equity stock and stock units forfeited (1)
113,108
Total shares available for grant as of March 31, 202412,014,121
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(1)    For purposes of determining the number of shares available for grant under the 2015 Plan against the maximum number of shares authorized, each restricted stock unit granted prior to April 27, 2023 reduces the number of shares available for grant by 1.5 shares and each restricted stock unit forfeited increases shares available for grant by 1.5 shares. Each restricted stock unit granted on or after April 27, 2023 reduces the number of shares available for grant by 1.0 shares and each restricted stock unit forfeited increases shares available for grant by 1.0 shares.
General Stock Option Information
The following table summarizes stock option activity under the Company’s equity incentive plans for the three months ended March 31, 2024 and information regarding stock options outstanding, exercisable, and vested and expected to vest as of March 31, 2024.
 Options Outstanding  
 (In thousands, except shares, per share amounts and years)
Number of
 Shares
Weighted-
 Average
 Exercise Price
 Per Share
Weighted-
 Average
 Remaining
 Contractual
 Term (years)
Aggregate
 Intrinsic
 Value
Outstanding as of December 31, 2023124,732$11.60   
Options exercised(25,125)$8.76  $1,333 
Options expired(1,125)$8.76   
Outstanding as of March 31, 202498,482$12.36 3.97$4,870 
Vested or expected to vest at March 31, 202498,482$12.36 3.97$4,870 
Options exercisable at March 31, 202498,482$12.36 3.97$4,870 
Employee Stock Purchase Plan
No purchases were made under the 2015 Employee Stock Purchase Plan (“2015 ESPP”) during the three months ended March 31, 2024 and 2023, respectively. As of March 31, 2024, approximately 2.4 million shares under the 2015 ESPP remained available for issuance.
Stock-Based Compensation
For the three months ended March 31, 2024 and 2023, the Company maintained stock plans covering a broad range of potential equity grants including stock options, nonvested equity stock and equity stock units and performance-based instruments. In addition, the Company sponsors the 2015 ESPP, whereby eligible employees are entitled to purchase common stock semi-annually, by means of limited payroll deductions, at a 15% discount from the fair market value of the common stock as of specific dates.
Stock Options
There were no stock options granted during the three months ended March 31, 2024 and 2023, respectively. All compensation cost net of expected forfeitures, related to unvested stock-based compensation arrangements granted under the stock option plans had been fully recognized as of December 31, 2023. There was no stock-based compensation expense related to stock options for the three months ended March 31, 2024. Stock-based compensation expense related to stock options was immaterial for the three months ended March 31, 2023.
Employee Stock Purchase Plan
For the three months ended March 31, 2024 and 2023, the Company recorded stock-based compensation expense related to the 2015 ESPP of $0.6 million for both periods. As of March 31, 2024, there was $0.2 million of total unrecognized compensation cost related to stock-based compensation arrangements granted under the 2015 ESPP. That cost is expected to be recognized over one month.
Nonvested Equity Stock and Stock Units
The Company grants nonvested equity stock units to officers, employees and directors. During the three months ended March 31, 2024, the Company granted nonvested equity stock units totaling approximately 0.1 million. During the three months ended March 31, 2023, the Company granted nonvested equity stock units totaling approximately 1.1 million. These awards have a service condition, generally a service period of four years, except in the case of grants to directors, for which the service period is one year. For the three months ended March 31, 2024, the nonvested equity stock units were valued at the date of grant giving them a fair value of approximately $3.6 million. For the three months ended March 31, 2023, the nonvested equity stock units were valued at the date of grant giving them a fair value of approximately $52.7 million. During the first quarter of 2024, the Company did not grant any performance unit awards. During the first quarter of 2023, the Company granted performance unit awards to certain company executive officers with vesting subject to the achievement of certain performance and/or market conditions. The ultimate number of performance units that can be earned can range from 0% to 200% of target depending on performance relative to target over the applicable period. The shares earned will vest on the third or fourth anniversary of the date of grant. The Company’s shares available for grant have been reduced to reflect the shares that could be earned at the maximum target.
For the three months ended March 31, 2024, the Company recorded stock-based compensation expense of approximately $8.9 million primarily related to all outstanding nonvested equity stock grants. For the three months ended March 31, 2023, the Company recorded stock-based compensation expense of approximately $12.5 million, related to all outstanding nonvested equity stock grants.
Unrecognized stock-based compensation related to all nonvested equity stock grants, net of estimated forfeitures, was approximately $57.9 million at March 31, 2024. This amount is expected to be recognized over a weighted-average period of 2.0 years.
The following table reflects the activity related to nonvested equity stock and stock units for the three months ended March 31, 2024:
Nonvested Equity Stock and Stock UnitsShares
Weighted-
 Average
 Grant-Date
 Fair Value
Nonvested at December 31, 20233,430,192$32.90 
Granted54,262$66.63 
Vested(1,063,195)$26.15 
Forfeited(76,473)$32.25 
Nonvested at March 31, 20242,344,786$36.75