Stock-based compensation (Tables)

12 Months Ended

Dec. 31, 2020

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,
	2020		2019		2018
Stock options	$	93,977	$	95,668	$	83,449
Restricted stock units	49,326		63,580		26,628
Employee stock purchase plan and other	13,328		1,381		759
	$	156,631	$	160,629	$	110,836

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,
	2020		2019		2018
Research and development	$	72,239	$	80,139	$	54,422
Selling, general and administrative	84,392		80,490		56,414
	$	156,631	$	160,629	$	110,836

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,
	2020		2019		2018
Expected volatility	69.5	%	70.7	%	75.5	%
Expected term (in years)	6.0		6.0		6.0
Risk-free interest rate	1.4	%	2.3	%	2.7	%
Expected dividend yield	0.0	%	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, 2019	5,483	$	116.30
Granted	1,585	$	69.40
Exercised	(95)	$	19.43
Canceled or forfeited	(711)	$	124.00
Outstanding at December 31, 2020	6,262	$	105.02	7.0	$	15,716
Exercisable at December 31, 2020	3,669	$	107.11	5.8	$	15,659
Vested and expected to vest at December 31, 2020	6,262	$	105.02	7.0	$	15,716

(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, 2020.

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, 2019	1,127	$	146.10
Granted	1,018	70.18
Vested	(433)	130.10
Forfeited	(217)	123.32
Unvested balance at December 31, 2020	1,495	$	102.34