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Stock-Based Awards (Tables)
3 Months Ended
Mar. 31, 2020
Share-based Payment Arrangement [Abstract]  
Summary of Stock Option Activity
The following table summarizes option award activity under the 2017 Plan and the 2015 Plan: 
Number of
Options
Weighted-
Average
Exercise Price
Weighted-
Average
remaining
contractual
life (years)
Aggregate
Intrinsic
Value (in thousands)
Balance at December 31, 201911,640,734  $12.96  7.79
Options granted
2,998,584  24.63  
Options exercised
(259,569) 7.57  
Options forfeited
(130,071) 18.02  
Balance at March 31, 202014,249,678  $15.47  8.03$64,492  
Options vested and expected to vest at March 31, 2020
12,504,946  $17.53  8.40$35,128  
Options exercisable at March 31, 20204,729,113  $12.63  7.42$29,163  
Summary of Assumptions Used for Estimating the Fair Value of Stock Granted
The estimated fair value of stock options granted to employees were calculated using the Black-Scholes option-pricing model using the following assumptions:
Three Months Ended March 31,
20202019
Expected term (in years)
6.086.08
Volatility
65.2% - 65.6%
69.3% - 69.4%
Risk-free interest rate
0.7% - 1.7%
2.5% - 2.6%
Dividend yield
Restricted Stock Activity
Aggregated information regarding RSAs and RSUs granted under the 2017 Plan for the three months ended March 31, 2020 is summarized below:
Share Awards & UnitsWeighted-Average Fair Value at Date of Grant per Share
Unvested at December 31, 2019882,636  $18.67  
Granted868,500  24.50  
Vested and released(91,839) 18.60  
Forfeited(21,313) 18.34  
Unvested at March 31, 20201,637,984  $21.75  
Expected to vest - March 31, 20201,637,984  $21.75  
Summary of Stock-Based Compensation Expense
The Company’s results of operations include expenses relating to stock-based compensation as follows (in thousands):
Three Months Ended March 31,
20202019
Research and development
$6,054  $3,982  
General and administrative
4,731  2,892  
Total
$10,785  $6,874