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Marketable Securities - Fair Value of Available for Sale Marketable Securities (Detail) - Level 2 [Member] - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Available for sale securities, Adjusted Cost $ 38,716 $ 34,520
Available for sale securities, Unrealized Gains 12  
Available for sale securities, Unrealized Losses (4) (23)
Available for sale securities, Fair Value 38,724 34,497
Corporate Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available for sale securities, Adjusted Cost 34,905 31,521
Available for sale securities, Unrealized Gains 11  
Available for sale securities, Unrealized Losses (4) (23)
Available for sale securities, Fair Value 34,912 31,498
Government-Sponsored Enterprises Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available for sale securities, Adjusted Cost 3,811 2,999
Available for sale securities, Unrealized Gains 1  
Available for sale securities, Fair Value $ 3,812 $ 2,999