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Warrants (Tables)
9 Months Ended
Sep. 30, 2018
Class of Warrant or Right [Line Items]  
Schedule of Stockholders' Equity Note, Warrants or Rights
Warrants outstanding and exercisable are summarized as follows:
 
 
 
Shares(1)
 
Weighted
Average
Exercise
Price
 
Range of
Exercise Prices
Outstanding at December 31, 2017
180,894

 
$
24.96

 
$2.50-$105.00
Exercised

 

 
Expired
(3,988
)
 
31.25

 
Outstanding at September 30, 2018
176,906

 
$
24.87

 
$2.50-$105.00
Exercisable at September 30, 2018
176,906

 
$
24.87

 
$2.50-$105.00
 
 
(1) Outstanding and exercisable information includes 3,000 equity-classified warrants.

Black Scholes  
Class of Warrant or Right [Line Items]  
Schedule Of Share Based Payment Award, Warrants, Valuation Assumptions
The assumptions used in the Black-Scholes option-pricing model to estimate the fair value of the warrant liability as of September 30, 2018 were as follows:
 
 
Expected volatility
95.0%-122.5%
Risk-free interest rate
2.6%-2.9%
Dividend yield
Expected life in years
1.0-3.2

Monte Carlo Simulation Model  
Class of Warrant or Right [Line Items]  
Schedule Of Share Based Payment Award, Warrants, Valuation Assumptions
The assumptions used in the Monte Carlo simulation model to estimate the fair value of the warrant liability as of September 30, 2018 were as follows:
 
 
Expected volatility
114.3%
Risk-free interest rate
2.6%
Dividend yield
Expected life in years
1.1