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Fair Value Assumptions Used in Probability Weighted Approach and Monte Carlo Simulation Model (Detail) - Series 1 Preferred Stock - $ / shares
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Preferred stock conversion limit - percentage of outstanding common stock 19.90% 19.90%
Preferred conversion floor price $ 1.00 $ 1.00
Measurement Input, Risk Free Interest Rate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value Assumptions of Preferred Stock 1.04%  
Measurement Input, Expected Dividend Rate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value Assumptions of Preferred Stock 0.00% 0.00%
Measurement Input, Price Volatility    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair Value Assumptions of Preferred Stock 70.50%