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Equity Incentive Plans and Stock-Based Compensation (Tables)
 9 Months Ended
Sep. 30, 2021
Share-based Payment Arrangement [Abstract]  
Schedule of shares available for grant
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, 202012,412,320
Stock options forfeited 51,477
Nonvested equity stock and stock units granted (1) (2)
(3,719,258)
Nonvested equity stock and stock units forfeited (1)
1,306,393
Total shares available for grant as of September 30, 202110,050,932
_________________________________________
(1)    For purposes of determining the number of shares available for grant under the 2015 Equity Incentive Plan (the “2015 Plan”) against the maximum number of shares authorized, each restricted stock granted reduces the number of shares available for grant by 1.5 shares and each restricted stock forfeited increases shares available for grant by 1.5 shares.
(2)    Amount includes approximately 0.4 million shares that have been reserved for potential future issuance related to certain performance unit awards granted in the first quarter of 2021 and discussed under the section titled “Nonvested Equity Stock and Stock Units” below.
Schedule of stock option activity
The following table summarizes stock option activity under the Company’s equity incentive plans for the nine months ended September 30, 2021 and information regarding stock options outstanding, exercisable, and vested and expected to vest as of September 30, 2021.
 Options Outstanding  
 (In thousands, except 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, 2020964,211$11.08   
Options granted—$—   
Options exercised(176,774)$10.97   
Options forfeited(51,477)$15.09   
Outstanding as of September 30, 2021735,960$10.83 4.4$8,368 
Vested or expected to vest at September 30, 2021735,700$10.83 4.4$8,366 
Options exercisable at September 30, 2021668,111$10.60 4.1$7,749 
Weighted-average assumptions for employee stock purchase plan
Employee Stock Purchase Plan
 Nine Months Ended
September 30,
 20212020
Employee Stock Purchase Plan:  
Expected stock price volatility32%46%
Risk free interest rate0.04%0.12%
Expected term (in years)0.50.5
Weighted-average fair value of purchase rights granted under the purchase plan$4.53$3.50
Schedule of nonvested equity stock and stock units activity
The following table reflects the activity related to nonvested equity stock and stock units for the nine months ended September 30, 2021:
Nonvested Equity Stock and Stock UnitsShares
Weighted-
 Average
 Grant-Date
 Fair Value
Nonvested at December 31, 20204,851,265$12.82 
Granted2,231,223$21.00 
Vested(1,438,169)$12.39 
Forfeited(686,279)$15.06 
Nonvested at September 30, 20214,958,040$16.32