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Marketable Securities and Fair Value Measurements - Changes in Fair Value of Level 3 Financial Liabilities (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Changes in fair value of the financial liabilities      
Decrease in fair value included in change in fair value of common stock warrants $ (807,000) $ (134,000) $ (722,000)
Significant Unobservable Inputs (Level 3)      
Changes in fair value of the financial liabilities      
Balance at beginning of year 1,167,614 1,301,138  
Decrease in fair value included in change in fair value of common stock warrants (806,819) (133,524)  
Balance at end of year $ 360,795 $ 1,167,614 $ 1,301,138