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Investments and Fair Value of Financial Instruments - Summary of Amortized Cost and Fair Value of the Available-For-Sale Investment Securities Portfolio (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]    
Amortized Cost $ 28,495 $ 12,842
Unrealized Gains 0 0
Unrealized Losses 0 (1)
Estimated Fair Value 28,495 12,841
Cash Equivalents [Member] | Commercial Paper [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Amortized Cost 4,095  
Unrealized Gains 0  
Unrealized Losses 0  
Estimated Fair Value 4,095  
Cash Equivalents [Member] | Money Market Funds [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Amortized Cost 5,469 8,943
Unrealized Gains 0 0
Unrealized Losses 0 0
Estimated Fair Value 5,469 8,943
Short-term Investments [Member] | Commercial Paper [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Amortized Cost 5,035 1,399
Unrealized Gains 0 0
Unrealized Losses 0 0
Estimated Fair Value 5,035 1,399
Short-term Investments [Member] | Treasury Bills [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Amortized Cost 11,106  
Unrealized Gains 0  
Unrealized Losses 0  
Estimated Fair Value 11,106  
Short-term Investments [Member] | U.S. Government Securities [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Amortized Cost 2,790 2,500
Unrealized Gains 0 0
Unrealized Losses 0 (1)
Estimated Fair Value $ 2,790 $ 2,499