Annual report pursuant to Section 13 and 15(d)

Stockholders' Equity - Equity Offerings (Details)

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Stockholders' Equity - Equity Offerings (Details) - USD ($)
 12 Months Ended
Sep. 16, 2015
Jul. 07, 2015
May. 08, 2015
Dec. 10, 2014
Mar. 06, 2014
Feb. 05, 2014
Dec. 31, 2015
Dec. 31, 2014
Equity Offerings                
Aggregate gross proceeds from shares sold             $ 90,306,634 $ 107,290,323
Proceeds from issuance of common stock, net             $ 90,310,634 $ 107,290,323
Common stock, shares authorized             100,000,000 100,000,000
Preferred Stock, shares authorized             5,000,000 5,000,000
Public Offering                
Equity Offerings                
Issuance of common stock in public offering (in shares)       11,250,000        
Issuance of common stock underwriters allotment option (in shares)       1,598,000        
Issuance of common stock (in shares) 7,475,000     12,848,000        
Offering price (in dollars per share) $ 9.75     $ 4.00        
Aggregate gross proceeds from shares sold $ 72,900,000     $ 51,400,000        
Proceeds from issuance of common stock, net 68,300,000              
Payment of underwriting discounts and commissions 4,400,000              
Offering expenses $ 200,000              
At-the-market sales facility                
Equity Offerings                
Issuance of common stock (in shares)   1,000,000 2,700,000          
Aggregate gross proceeds from shares sold   $ 6,000,000 $ 16,900,000          
Proceeds from issuance of common stock, net   5,800,000 16,200,000          
Payment of underwriting discounts and commissions   $ 200,000 $ 500,000          
At-the-market sales facility | Weighted-average                
Equity Offerings                
Offering price (in dollars per share)   $ 6.0001 $ 6.2503          
Initial public offering                
Equity Offerings                
Issuance of common stock underwriters allotment option (in shares)         270,449      
Issuance of common stock (in shares)           9,250,000    
Offering price (in dollars per share)         $ 7.00 $ 7.00    
Aggregate gross proceeds from shares sold         $ 1,900,000 $ 64,800,000