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Stock Based Compensation
 6 Months Ended
Jun. 30, 2020
Equity [Abstract]  
Stock-based Compensation Stock-Based Compensation
The Company recorded approximately $2.6 million and $2.3 million in stock-based compensation expense for the three months ended June 30, 2020 and 2019, respectively, and approximately $4.8 million and $4.5 million in stock-based compensation expense for the six month periods ended June 30, 2020 and 2019, respectively.
A summary of the stock option activity under the Company’s plans for the six months ended June 30, 2020 is as follows:
 
Number of
Shares		Weighted Average Exercise Price
Options outstanding at December 31, 20195,318,759  $9.97  
Options granted816,428  $28.09  
Less:
Options exercised691,299  $7.42  
Options forfeited57,659  $14.72  
Options outstanding at June 30, 20205,386,229  $13.01  
Exercisable at June 30, 20203,101,808  $8.66  
The fair value of each stock option award granted during the six months ended June 30, 2020 was estimated as of the grant date using a trinomial lattice model with the following weighted average assumptions:
 Six Months Ended
June 30, 2020
Expected term (in years)
4.0 - 5.5
Risk-free interest rate (%)0.7%
Expected volatility (%)
39.9% - 44.5%
Dividend yield (%)—
Weighted average fair value/share at grant date$8.81
 
As of June 30, 2020, there was approximately $8.8 million of unrecognized stock-based compensation expense related to stock options that will be recognized over a weighted-average period of approximately 1.46 years.

A summary of the restricted stock activity under the Company’s plans for the six months ended June 30, 2020 is as follows:
 
Number of Restricted
Shares		Weighted Average Grant Date Fair Value
Nonvested at December 31, 2019335,298  $15.75  
Granted143,177  $28.15  
Vested(166,836) $12.81  
Forfeited(4,919) $20.34  
Nonvested at June 30, 2020306,720  $23.06  

As of June 30, 2020, there was approximately $5.0 million of unrecognized stock-based compensation expense related to restricted stock that will be recognized over a weighted-average period of approximately 1.48 years.
Employee Stock Purchase Plan (“ESPP”)
The Company offers an ESPP through which eligible employees may purchase shares of our common stock at a discount of 15% of the fair market value of the Company’s common stock. 
During the three months ended June 30, 2020 and 2019, employees purchased 41,058 and 37,255 shares, respectively, under the ESPP. The expense recorded for each of these periods was approximately $0.2 million. During the six months ended June 30, 2020 and 2019, employees purchased 75,388 and 73,287 shares, respectively, under the ESPP. The expense recorded for these periods was approximately $0.4 million and $0.3 million, respectively.