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Stock-Based Awards (Tables)
 12 Months Ended
Dec. 31, 2019
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, 20189,612,652  $10.49  8.20$97,804  
Options granted3,610,835  18.58  
Options exercised(786,638) 4.27  
Options forfeited(796,115) 16.72  
Balance at December 31, 201911,640,734  $12.96  7.79$51,879  
Options vested and expected to vest at December 31, 20199,896,002  $15.13  8.17$22,672  
Options exercisable at December 31, 20193,938,523  $11.19  7.38$24,552  
Summary of Assumptions Used for Estimating the Fair Value of Stock Granted
Stock Options Granted to Employees with Service-Based Vesting
The estimated fair value of stock options granted to employees were calculated using the Black-Scholes option-pricing model using the following assumptions:
Year Ended December 31,
201920182017
Expected term (in years)5.50 - 6.08  5.50 - 6.086.08
Volatility65.5% - 77.8%  79.4% - 87.4%  81.9% - 91.3%
Risk-free interest rate1.5% - 2.6%  2.6% - 3.1%  1.8% - 2.3%  
Dividend yield—  —  —  
Restricted Stock Activity
Aggregated information regarding RSAs and RSUs granted under the Plan for the year ended December 31, 2019 is summarized below:
Share Awards & UnitsWeighted-Average Fair Value at Date of Grant per Share
Unvested at December 31, 2018503,243  $4.86  
Granted850,234  18.86  
Vested and released(419,064) 2.63  
Forfeited(51,777) 20.50  
Unvested at December 31, 2019882,636  $18.67  
Expected to vest – December 31, 2019882,636  $18.67  
Summary of Assumptions Used for Estimating the Fair Value of Stock Purchase Rights Granted in ESPP
The estimated fair value of stock purchase rights granted under the ESPP were calculated using the Black-Scholes option-pricing model using the following assumptions:
Year Ended December 31,
201920182017
Expected term (in years)0.50 - 1.00  0.50 - 1.00  0.48 - 0.98  
Volatility47.6% - 59.1%  61.0% - 64.3%  50.0% - 51.3%  
Risk-free interest rate1.6% - 2.4%  2.1% - 2.7%  1.5% - 1.7%  
Dividend yield—  —  —  
Summary of Stock-Based Compensation Expense
The Company’s results of operations include expenses relating to stock-based compensation as follows (in thousands):
Year Ended December 31,
201920182017
Research and development$19,261  $10,093  $2,852  
General and administrative19,117  8,698  1,557  
Total$38,378  $18,791  $4,409