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Long-term debts, net - Schedule of Assumptions Used to Estimate Fair Value of Warrants (Detail)
12 Months Ended
Dec. 31, 2017
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Expected volatility 82.50%
Expected dividend yield 0.00%
Minimum  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Expected term (in years) 29 days
Risk-free interest rate 1.02%
Maximum  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Expected term (in years) 9 years
Risk-free interest rate 2.35%