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Stock Based Compensation
 9 Months Ended
Sep. 30, 2020
Equity [Abstract]  
Stock-based Compensation Stock-Based Compensation
The Company recorded approximately $2.7 million and $3.3 million in stock-based compensation expense for the three months ended September 30, 2020 and 2019, respectively, and approximately $7.5 million and $7.7 million in stock-based compensation expense for the nine month periods ended September 30, 2020 and 2019, respectively.
A summary of the stock option activity under the Company’s plans for the nine months ended September 30, 2020 is as follows:
 
Number of
Shares		Weighted Average Exercise Price
Options outstanding at December 31, 20195,318,759 $9.97 
Options granted845,120 $28.33 
Less:
Options exercised1,276,144 $7.75 
Options forfeited63,169 $15.82 
Options outstanding at September 30, 20204,824,566 $13.72 
Exercisable at September 30, 20202,552,710 $8.86 
The fair value of each stock option award granted during the nine months ended September 30, 2020 was estimated as of the grant date using a trinomial lattice model with the following weighted average assumptions:
 Nine Months Ended
September 30, 2020
Expected term (in years)
4.0 - 5.5
Risk-free interest rate (%)0.7%
Expected volatility (%)
39.9% - 44.6%
Dividend yield (%)—
Weighted average fair value/share at grant date$8.88
 
As of September 30, 2020, there was approximately $7.3 million of unrecognized stock-based compensation expense related to stock options that will be recognized over a weighted-average period of approximately 1.93 years.

A summary of the restricted stock activity under the Company’s plans for the nine months ended September 30, 2020 is as follows:
Number of Restricted
Shares		Weighted Average Grant Date Fair Value
Nonvested at December 31, 2019335,298 $15.75 
Granted149,012 $28.44 
Vested(183,532)$12.88 
Forfeited(5,540)$21.10 
Nonvested at September 30, 2020295,238 $23.78 

As of September 30, 2020, there was approximately $4.2 million of unrecognized stock-based compensation expense related to restricted stock that will be recognized over a weighted-average period of approximately 1.90 years.
Employee Stock Purchase Plan (“ESPP”)
The Company offers an ESPP through which eligible employees may purchase shares of the Company's common stock at a discount of 15% of the fair market value of the Company’s common stock. 
During the three months ended September 30, 2020 and 2019, employees purchased 29,853 and 28,672 shares, respectively, under the ESPP. The expense recorded for these periods was approximately $0.2 million and $0.1 million, respectively. During the nine months ended September 30, 2020 and 2019, employees purchased 105,241 and 101,959 shares, respectively, under the ESPP. The expense recorded for these periods was approximately $0.6 million and $0.4 million, respectively.