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Stock-based compensation (Tables)
6 Months Ended
Jun. 30, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of stock option activity and related information
A summary of our stock option activity and related information is as follows:
 
 
Shares
 
Weighted
average
exercise
price per
share
 
Weighted
Average
Remaining
contractual
Term
(years)
 
Aggregate
intrinsic
value
(in thousands)
Outstanding at December 31, 2015
4,650,114

 
$
21.49

 
8.58
 
$
2,970

Granted
1,487,100

 
7.48

 
 
 
 

Exercised
(24,663
)
 
4.52

 
 
 
 

Forfeited / Canceled
(844,303
)
 
32.89

 
 
 
 

Outstanding at June 30, 2016
5,268,248

 
15.79

 
8.44
 
278

Exercisable at June 30, 2016
2,052,413

 
17.49

 
7.62
 
278

Vested and expected to vest at June 30, 2016
4,542,808

 
15.96

 
8.37
 
278

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
The fair value of each stock-based option award is estimated on the grant date using the Black-Scholes option pricing model using the following assumptions:
 
Three months ended June 30,
 
Six months ended June 30,
 
2016
 
2015
 
2016
 
2015
Risk-free interest rate
1.4% - 1.5%
 
1.6% - 2.3%
 
1.4% - 2.0%
 
1.6% - 2.3%
Dividend yield
 
 
 
Volatility
86% - 89%
 
73%
 
78% - 89%
 
73% - 75%
Expected term (years)
5.3 - 6.1
 
5.3 - 9.9
 
5.3 - 9.9
 
5.3 - 9.9