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Stock-based compensation (Tables)
 12 Months Ended
Dec. 31, 2019
Share-based Payment Arrangement [Abstract]  
Summary of Stock-Based Compensation Expense by Award Type Stock-based compensation expense recognized by award type is as follows (in thousands):
Year Ended December 31,
201920182017
Stock options$95,668  $83,449  $42,262  
Restricted stock units63,580  26,628  10,495  
Employee stock purchase plan1,381  759  525  
$160,629  $110,836  $53,282  
Schedule of Stock-Based Compensation Expense by Classification
Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive loss was as follows (in thousands):
Year Ended December 31,
201920182017
Research and development$80,139  $54,422  $26,633  
Selling, general and administrative80,490  56,414  26,649  
$160,629  $110,836  $53,282  
Assumptions Used for the Black-Scholes Option-Pricing Model to Determine the Per Share Weighted Average Fair Value for Options Granted
The fair value of each option issued to employees was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:
Year Ended December 31,
201920182017
Expected volatility70.7 %75.5 %78.1 %
Expected term (in years)6.06.06.0
Risk-free interest rate2.3 %2.7 %2.1 %
Expected dividend yield0.0 %0.0 %0.0 %
Summary of Stock Option Activity Under Plan
The following table summarizes the stock option activity under the Company’s equity awards plans:
Shares
(in thousands)		Weighted-
average
exercise price
per share		Weighted-
average
contractual
life
(in years)		Aggregate
intrinsic
value (a)
(in thousands)
Outstanding at December 31, 20184,643  $108.56  
Granted1,594  $129.76  
Exercised(354) $50.36  
Canceled or forfeited(400) $138.61  
Outstanding at December 31, 20195,483  $116.30  7.3$68,511  
Exercisable at December 31, 20192,853  $94.77  6.0$64,899  
Vested and expected to vest at December 31, 20195,483  $116.30  7.3$68,511  
(a)The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at December 31, 2019.
Summary of Restricted Common Stock Awards The following table summarizes the restricted stock unit activity under the Company’s equity award plans:
Shares
(in thousands)		Weighted-average
grant date
fair value
Unvested balance at December 31, 2018931  $155.99  
Granted570  128.87  
Vested(251) 138.93  
Forfeited(123) 155.68  
Unvested balance at December 31, 20191,127  $146.10