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Stock-Based Compensation (Tables)
 6 Months Ended
Jun. 30, 2020
Share-based Payment Arrangement [Abstract]  
Schedule of compensation costs recognized
The Company recognized stock-based compensation expense related to the 2003 Stock Option and Restricted Stock Plan (the 2003 Plan), the 2013 Equity Incentive Plan (the 2013 Plan), and the 2013 Employee Stock Purchase Plan (the 2013 ESPP) in the consolidated statements of operations and comprehensive loss during the three and six months ended June 30, 2020 and 2019, respectively, as follows (in thousands):
 Three Months Ended 
 
June 30,		Six Months Ended 
 
June 30,
 2020201920202019
Research and development$3,259  $2,006  $6,400  $5,505  
Selling, general and administrative3,881  3,006  7,419  6,499  
 $7,140  $5,012  $13,819  $12,004  
Schedule of weighted-average assumptions used for estimating fair value
The fair value of each option issued to employees was estimated on the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:
Three Months Ended 
 
June 30,		Six Months Ended 
 
June 30,
 2020201920202019
Expected volatility57.9 %58.3 %55.4 %59.0 %
Expected term (in years)6.06.06.06.0
Risk-free interest rate0.4 %2.2 %1.5 %2.6 %
Expected dividend yield— %— %— %— %
Summary of stock option activity The following table summarizes the stock option activity under the Company’s stock option plans during the six months ended June 30, 2020 (in thousands, except per share amounts and years):
 Number
of Stock Options		Weighted-
Average
Exercise
Price
Per Share		Weighted-
Average
Contractual
Life (in years)		Aggregate
Intrinsic
Value(1)
Outstanding at December 31, 20193,820  $36.26  6.81 
Granted926  $58.36    
Exercised(913) $30.77    
Canceled or forfeited(153) $42.20    
Outstanding at June 30, 20203,680  $42.94  7.48$192,690  
Exercisable at June 30, 20201,882  $36.53  6.18$110,552  
(1) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at June 30, 2020.
Schedule of nonvested restricted stock units activity
The following table summarizes the restricted stock unit (RSU) activity under the 2013 Plan during the six months ended June 30, 2020 (in thousands, except per share amounts):
 Number
of Stock Units		Weighted-
Average
Grant Date Fair Value Per Share
Unvested balance at December 31, 2019397  $39.20  
Granted214  57.25  
Vested(123) 37.76  
Forfeited(31) 42.57  
Unvested balance at June 30, 2020457  $47.80  
Share-based compensation, performance shares award nonvested activity The following table summarizes PSU activity under the 2013 Plan during the six months ended June 30, 2020 (in thousands, except per share amounts):
 Number
of Stock Units		Weighted-
Average
Grant Date Fair Value Per Share
Unvested balance at December 31, 2019—  $—  
Granted (1)78  52.99  
Vested—  —  
Forfeited(3) 52.99  
Unvested balance at June 30, 202075  $52.99  
(1)Pursuant to the terms of the awards granted on January 22, 2020, the actual number of awards earned could range between 0% and 200% of the number of awards granted.