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Marketable Securities and Fair Value Measurements - Assumptions Used to Estimate Fair Value of Common Stock Warrants (Details) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Fair Value Disclosures [Abstract]    
Risk-free interest rate 1.75% 1.10%
Expected volatility 55.00% 61.00%
Expected life in years 8 months 12 days 1 year 8 months 12 days
Dividend yield $ 0 $ 0