Stock-based compensation (Tables)

3 Months Ended

Mar. 31, 2019

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

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,
	2019		2018
Stock options	$	23,183	$	17,295
Restricted stock units		8,881		5,541
Employee stock purchase plan		277		159
	$	32,341	$	22,995

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,
	2019		2018
Research and development	$	15,516	$	11,624
General and administrative		16,825		11,371
	$	32,341	$	22,995

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, 2018		4,643		$	108.56
Granted		1,085		$	135.53
Exercised		(189	)	$	50.31
Canceled, forfeited, or expired		(155	)	$	136.45
Outstanding at March 31, 2019		5,384		$	115.84
Exercisable at March 31, 2019		2,372		$	82.62
Vested and expected to vest at March 31, 2019		5,382		$	115.85

Restricted Stock Units [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

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, 2018		931		$	155.99
Granted		368		$	135.70
Vested		(131	)	$	131.06
Forfeited		(40	)	$	149.31
Unvested balance at March 31, 2019		1,128		$	152.49