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Fair Value Measurement of Assets and Liabilities - Summary of Company's Financial Instruments, Fair Value on a Recurring Basis (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of financial instruments, recurring $ 0 $ 0
Convertible Debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Principal 105,125 105,125
Fair Value, Measurements, Nonrecurring | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of debt $ 86,800 $ 84,800