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Fair Value Assumptions Used in Probability Weighted Approach and Monte Carlo Simulation Model (Detail) - Series 1 Preferred Stock - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Risk-free interest rate 1.04%  
Expected dividend rate 0.00% 0.00%
Expected volatility 70.50%  
Preferred stock conversion limit-percentage of outstanding common stock 19.90% 19.90%
Preferred conversion floor price $ 1.00 $ 1.00