Stock-based compensation (Tables)

3 Months Ended

Mar. 31, 2020

Share-based Payment Arrangement [Abstract]

Summary of Stock-Based Compensation Expense by Award Type

Stock-based compensation expense by award type included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands):

	For the three months ended March 31,
	2020	2019
Stock options	24,440	23,183
Restricted stock units	11,853	8,881
Employee stock purchase plan	—	277
	36,293	32,341

Schedule of Stock-Based Compensation Expense by Classification

Stock-based compensation expense by classification included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands): 

	For the three months ended March 31
	2020	2019
Research and development	16,269	15,516
Selling, general and administrative	20,024	16,825
	36,293	32,341

Summary of Stock Option Activity Under Plan

The following table summarizes the stock option activity under the Company’s equity award plans:

	Shares (in thousands)	Weighted- average exercise price per share
Outstanding at December 31, 2019	5,483	$	116.30
Granted	1,147	$	75.65
Exercised	(20)	$	37.71
Canceled, forfeited, or expired	(132)	$	148.55
Outstanding at March 31, 2020	6,478	$	108.68
Exercisable at March 31, 2020	3,248	$	99.92
Vested and expected to vest at March 31, 2020	6,478	$	108.68

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	709	$	77.69
Vested	(204)	$	132.15
Forfeited	(47)	$	150.73
Unvested balance at March 31, 2020	1,585	$	117.15