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Marketable Securities and Fair Value Measurements - Narrative (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Jun. 30, 2015
Marketable Securities and Fair Value Measurements (Textual) [Abstract]            
Carryover basis         $ 0 $ 0
Stock warrants exercised (shares)     0 0    
Common stock warrants $ 228,665   $ 228,665   $ 360,795  
Decrease attributable to change in fair value of common stock warrants $ (260,000) $ 313,000 $ (132,000) $ 196,000    
Using Quoted Prices in Active Markets for Identical Assets (Level 1)            
Marketable Securities and Fair Value Measurements (Textual) [Abstract]            
Number of shares held in an affiliated entity (shares) 1,644,155   1,644,155      
Using Quoted Prices in Active Markets for Identical Assets (Level 1) | Common Stock            
Marketable Securities and Fair Value Measurements (Textual) [Abstract]            
Investment owned (shares) 395,758   395,758