Stock-based compensation (Tables)

3 Months Ended

Mar. 31, 2014

Stock-based compensation

Summary of Founders' stock activity and related information

	Shares
Unvested at December 31, 2013	987,081
Granted	—
Vested	(164,515	)
Unvested at March 31, 2014	822,566

Summary of stock option activity and related information

				Weighted
			Weighted	average
			average	remaining
			exercise	contractual	Aggregate
			price per	term	intrinsic
	Shares		Share ($)	(years)	value
					(in thousands)
Outstanding at December 31, 2013	2,413,237		9.26	9.05	6,087
Granted	377,098		10.09
Exercised	(109,866	)	0.04
Forfeited	(510,440	)	12.87
Cancelled	—		—
Outstanding at March 31, 2014	2,170,029		8.95	8.67	3,753
Exercisable at March 31, 2014	537,544		3.46	8.28	3,112
Vested and expected to vest at March 31, 2014	1,400,912		8.37	8.35	3,094

Schedule of assumptions used to estimate fair value of each stock-based option award on the grant date using the Black-Scholes option pricing model

	Three months ended March 31,
	2014	2013
Risk-free interest rate	1.6% - 1.8%	0.9% - 1.1%
Dividend yield	—	—
Volatility	78% - 84%	89% - 91%
Expected term (years)	5.3 - 5.8	5.3 - 9.7