Stock-based compensation (Tables)

6 Months Ended

Jun. 30, 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:

	For the				For the
	three months ended June 30,				six months ended June 30,
(in thousands)	2019		2018		2019		2018
Stock options	$	24,800	$	20,932	$	47,983	$	38,227
Restricted stock units		30,012		6,953		38,893		12,494
Employee stock purchase plan		299		171		576		330
	$	55,111	$	28,056	$	87,452	$	51,051

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: 

	For the				For the
	three months ended June 30,				six months ended June 30,
(in thousands)	2019		2018		2019		2018
Research and development	$	29,694	$	14,196	$	45,210	$	25,820
General and administrative		25,417		13,860		42,242		25,231
	$	55,111	$	28,056	$	87,452	$	51,051

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		1,266		$	136.17
Exercised		(282	)	$	47.80
Canceled, forfeited, or expired		(234	)	$	129.85
Outstanding at June 30, 2019		5,393		$	117.30
Exercisable at June 30, 2019		2,520		$	88.94
Vested and expected to vest at June 30, 2019		5,393		$	117.30

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		438		$	136.32
Vested		(197	)	$	142.82
Forfeited		(69	)	$	153.94
Unvested balance at June 30, 2019		1,103		$	150.67