Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2015

Stock-based Compensation

Schedule of weighted-average assumptions used in estimation of grant date fair values

	Years Ended December 31,
	2015			2014			2013
Expected term (years)		6.08			6.06			6.25
Volatility		55	%		59	%		62	%
Expected dividend yield		0	%		0	%		0	%
Risk-free interest rates		1.72	%		2.06	%		2.45	%

Summary of stock option activity

A summary of stock option activity for the year ended December 31, 2015 is as follows (in thousands, except for per share and weighted-average contractual life amounts):

	Shares			Weighted-Average Exercise Price (in dollars per share)		Weighted-Average Contractual Life (In Years)		Aggregate Intrinsic Value
Options outstanding at December 31, 2014		3,220		$	13.58
Granted		1,487			57.75
Exercised		(268	)		8.73
Cancelled		(31	)		17.31
Expired		—			—
Options outstanding at December 31, 2015		4,408		$	28.75		8.24	$	151,544
Options exercisable at December 31, 2015		1,738		$	12.21		6.96	$	85,747
Options vested or expected to vest at December 31, 2015		4,316		$	28.43		8.22	$	149,570

Summary of stock-based compensation expense by financial statement line

        The following table summarizes stock-based compensation expense by financial statement line (in thousands):

	Years Ended December 31,
	2015		2014		2013
Research and development	$	7,864	$	1,953	$	302
General and administrative		6,870		5,117		1,206
Share-based compensation expense included in operating expenses	$	14,734	$	7,070	$	1,508