Stock-based compensation (Tables)

3 Months Ended

Mar. 31, 2018

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,
	2018		2017
Stock options	$	17,295	$	9,342
Restricted stock units		5,541		1,956
Employee stock purchase plan		159		183
	$	22,995	$	11,481

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,
	2018		2017
Research and development	$	11,624	$	5,654
General and administrative		11,371		5,827
	$	22,995	$	11,481

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, 2017		3,755		$	67.91
Granted		1,009		$	203.72
Exercised		(301	)	$	65.53
Canceled or forfeited		(17	)	$	107.83
Outstanding at March 31, 2018		4,446		$	98.91
Exercisable at March 31, 2018		1,844		$	52.56
Vested and expected to vest at March 31, 2018		4,437		$	98.91

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, 2017		477		$	80.72
Granted		522		$	204.44
Vested		(74	)	$	63.26
Forfeited		(5	)	$	128.61
Unvested balance at March 31, 2018		920		$	151.94