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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2019
Share-based Payment Arrangement [Abstract]  
Summary of Assumptions used to Determine the Fair Value of Options Granted to Employees and Directors
The assumptions that the Company used to determine the fair value of options granted to employees and directors were as follows, presented on a weighted average basis:
Year Ended December 31,
201920182017
Risk-free interest rate2.2 %2.8 %1.9 %
Expected term (in years)6.26.16.0
Expected volatility74.5 %73.3 %78.3 %
Expected dividend yield0 %0 %0 %
Summary of Option Activity
The following table summarizes the Company's option activity from January 1, 2019 to December 31, 2019:
Number of SharesWeighted Average Exercise PriceWeighted Average Remaining Contractual Term
(in years)		Aggregate Intrinsic Value
(in thousands)
Outstanding as of December 31, 20185,787,151  $10.92  
Granted3,067,718  $28.77  
Exercised(1,504,583) $4.49  
Forfeited/Expired(599,347) $23.40  
Outstanding as of December 31, 20196,750,939  $19.36  8.0$289,615  
Options vested and expected to vest as of December 31, 20196,750,939  $19.36  8.0$289,615  
Options exercisable as of December 31, 20193,052,876  $10.62  6.9$157,592  
Summary of RSU Activity
The table below summarizes the Company's time-based restricted stock unit activity from January 1, 2019 to December 31, 2019:
Number
of Shares		Weighted
Average
Grant Date
Fair Value
Unvested at December 31, 201825,000  $33.43  
Granted87,000  $29.03  
Vested(12,500) $33.43  
Forfeited—  $—  
Unvested at December 31, 201999,500  $29.58  
Classification of Stock-Based Compensation Expense
Stock-based compensation expense was classified in the statements of operations and comprehensive loss as follows:
 Year Ended December 31,
(in thousands)201920182017
Research and development$7,934  $4,021  $1,320  
Selling, general, and administrative12,476  5,667  3,546  
Total stock-based compensation$20,410  $9,688  $4,866  
The following table summarizes share-based compensation expense associated with each of our share-based compensation arrangements:
Year Ended December 31,
(in thousands)201920182017
Stock options$19,328  $9,578  $4,866  
Time-based restricted stock units1,082  110  —  
Total stock-based compensation expense$20,410  $9,688  $4,866