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Marketable Investments - Summary of Gross Unrealized Losses and Market Value of Available-for-Sale Securities with Unrealized Losses (Detail)
$ in Thousands
Dec. 31, 2017
USD ($)
Schedule of Available-for-sale Securities [Line Items]  
Available for sale securities, Less Than 12 Months, Market Value $ 31,723
Available for sale securities, Less Than 12 Months, Unrealized Losses 149
Available for sale securities, 12 Months or Greater, Market Value 22,610
Available for sale securities, 12 Months or Greater, Unrealized Losses 39
Corporate Obligations [Member]  
Schedule of Available-for-sale Securities [Line Items]  
Available for sale securities, Less Than 12 Months, Market Value 31,723
Available for sale securities, Less Than 12 Months, Unrealized Losses 149
Available for sale securities, 12 Months or Greater, Market Value 20,817
Available for sale securities, 12 Months or Greater, Unrealized Losses 32
Federal Agency Obligations [Member]  
Schedule of Available-for-sale Securities [Line Items]  
Available for sale securities, 12 Months or Greater, Market Value 1,793
Available for sale securities, 12 Months or Greater, Unrealized Losses $ 7