Quarterly report pursuant to Section 13 or 15(d)

Investments and Fair Value Measurements - Category of Financial Instruments Not Held at Fair Value (Detail)

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Investments and Fair Value Measurements - Category of Financial Instruments Not Held at Fair Value (Detail) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Carrying Amount    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Financial Assets, Investments $ 16 $ 626
Carrying Amount | Level I    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Financial Assets, Cash and cash equivalents 57,715 53,822
Carrying Amount | Level II    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Financial liabilities, Notes Payable   740
Fair Value    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Financial Assets, Investments 16 626
Fair Value | Level I    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Financial Assets, Cash and cash equivalents $ 57,715 53,822
Fair Value | Level II    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Financial liabilities, Notes Payable   $ 740