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Stock-Based Compensation (Tables)
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
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 as of June 30, 2018 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, 2017
5,745,815

 
$
7.28

 
8.32
 
$
43

Granted
1,179,877

 
0.96

 
 
 
 

Forfeited/Canceled
(1,529,717
)
 
5.90

 
 
 
 

Outstanding at June 30
5,395,975

 
6.28

 
8.05
 
1

Exercisable at June 30, 2018
2,576,138

 
10.88

 
6.96
 
1

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, 2016
4,611,392

 
$
14.42

 
8.23
 
$
45

Granted
4,386,856

 
1.51

 
 
 
 
Forfeited / Canceled
(3,252,433
)
 
9.63

 
 
 
 
Outstanding at December 31, 2017
5,745,815

 
7.28

 
8.32
 
43

Exercisable at December 31, 2017
2,370,335

 
13.37

 
7.08
 
42

Vested and expected to vest at December 31, 2017
5,745,815

 
7.28

 
8.32
 
43

Schedule of assumptions used to estimate fair value of each stock option
The fair value of each stock-based option award is estimated on the grant date using the Black-Scholes option pricing model with the following assumptions:
 
Three months ended June 30,
 
Six months ended June 30,
 
2018
 
2017
 
2018
 
2017
Risk-free interest rate
 
1.3% - 2.0%
 
2.7%
 
1.3% - 2.2%
Dividend yield
 
 
 
Volatility
 
89% - 109%
 
83% - 85%
 
89% - 109%
Expected term (years)

 
1.8 - 6.9
 
6.1
 
1.8 - 6.9
The fair value of each stock option is estimated using the Black-Scholes option pricing model using the following assumptions:
 
December 31,

2017
 
2016
 
2015
Risk-free interest rate
1.3%-2.2%
 
1.3%-2.0%
 
1.6%-2.3%
Dividend yield
 
 
Volatility
87%-109%
 
78%-89%
 
72%-78%
Expected term (years)
2.0-6.9
 
5.3-9.9
 
5.3-9.9
Schedule of restricted stock unit activity  
A summary of our restricted stock unit ("RSU") activity and related information is as follows:
 
Shares
 
Weighted average grant date fair value
Outstanding at December 31, 2016
50,000

 
$
7.15

Granted

 

Vested

 

Forfeited
(50,000
)
 
7.15

Outstanding at December 31, 2017

 
$

Summary of awards  
The following table summarizes the December 9, 2014 award.
 
Award Type
Number of
RSUs Granted
 
Grant Date
Fair Value
 
RSUs Vested
as of December 31, 2015
Service-based
30,902

 
$
32.36

 
15,450

Performance-based - Year 1
11,588

 
$
43.47

 
4,635

Performance-based - Year 2
11,588

 
$