Stock-based compensation (Tables)

12 Months Ended

Dec. 31, 2016

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

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,
	2016		2015		2014
Stock options	$	33,966	$	37,536	$	9,487
Restricted stock awards		—		—		52
Restricted stock units		5,374		3,325		1,158
Employee stock purchase plan		416		259		66
	$	39,756	$	41,120	$	10,763

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,
	2016		2015		2014
Research and development	$	19,690	$	24,854	$	5,151
General and administrative		20,066		16,266		5,612
	$	39,756	$	41,120	$	10,763

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,
	2016			2015			2014
Expected volatility		74.3	%		72.6	%		82.3	%
Expected term (in years)		6.0			5.9			6.0
Risk-free interest rate		1.5	%		1.7	%		1.8	%
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 and aggregate intrinsic value in thousands):

	Shares			Weighted-average exercise price per share		Weighted-average contractual life (in years)		Aggregate intrinsic value (a) (inthousands)
Outstanding at December 31, 2015		3,532		$	48.74
Granted		1,047		$	52.09
Exercised		(377	)	$	16.30
Canceled or forfeited		(467	)	$	55.04
Outstanding at December 31, 2016		3,735		$	52.17		7.5	$	93,081
Exercisable at December 31, 2016		1,926		$	39.23		6.5	$	70,991
Vested and expected to vest at December 31, 2016		3,735		$	52.17		7.5	$	93,081

(a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at December 31, 2016.

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):

	Shares			Weighted-average grant date fair value
Unvested balance at December 31, 2015		148		$	65.79
Granted		263			52.12
Vested		(113	)		46.17
Forfeited		(35	)		46.65
Unvested balance at December 31, 2016		263		$	63.07