Stock-based Compensation (Tables)

3 Months Ended

12 Months Ended

Mar. 31, 2014

Dec. 31, 2013

Stock-Based Compensation

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

	Year Ended December 31,
	2013			2012			2011
Expected term (years)		6.25			6.25			6.25
Volatility		62	%		60	%		60	%
Expected dividend yield		0	%		0	%		0	%
Risk-free interest rates		2.45	%		1.10	%		1.35	%

Summary of stock option activity

A summary of stock option activity during the three months ended March 31, 2014 is as follows (in thousands, except for per share amounts):

	Shares			Weighted-Average Exercise Price (in dollars per share)		Weighted-Average Contractual Life (in years)		Aggregate Intrinsic Value
Options outstanding at December 31, 2013		1,667		$	7.05
Granted		1,144			7.80
Exercised		—			—
Forfeited		(116	)		9.00
Options outstanding at March 31, 2014		2,695		$	7.28		8.08	$	6,607
Options exercisable at March 31, 2014		1,142		$	6.31		6.41	$	3,916
Options vested or expected to vest at March 31, 2014		2,608		$	7.26		8.04	$	6,456

	Shares			Weighted-Average Exercise Price (in dollars per share)		Weighted-Average Contractual Life (In Years)		Aggregate Intrinsic Value
Options outstanding at December 31, 2012		1,710		$	7.07
Granted		1,064			7.87
Exercised		(5	)		2.51
Forfeited		(1,097	)		7.91
Expired		(5	)		3.42
Options outstanding at December 31, 2013		1,667		$	7.05		7.07	$	2,283
Options exercisable at December 31, 2013		1,086		$	6.22		6.60	$	2,231
Options vested or expected to vest at December 31, 2013		1,635		$	7.02		7.06	$	2,280

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,
	2013		2012		2011
Research and development	$	302	$	338	$	118
General and administrative		1,206		1,457		186
Share-based compensation expense included in operating expenses	$	1,508	$	1,795	$	304