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Marketable Investments - Summary of Gross Unrealized Losses and Market Value of Available-for-Sale Securities with Unrealized Losses (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]    
Available for sale securities, Less Than 12 Months, Market Value $ 35,843 $ 27,146
Available for sale securities, Less Than 12 Months, Unrealized Losses 165 43
Available for sale securities, 12 Months or Greater, Market Value 9,092 8,970
Available for sale securities, 12 Months or Greater, Unrealized Losses 1,908 2,030
State and Municipal Obligations [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Available for sale securities, Less Than 12 Months, Market Value    9,430
Available for sale securities, Less Than 12 Months, Unrealized Losses    14
Available for sale securities, 12 Months or Greater, Market Value      
Available for sale securities, 12 Months or Greater, Unrealized Losses      
Federal Agency and Corporate Obligations [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Available for sale securities, Less Than 12 Months, Market Value 35,843 17,716
Available for sale securities, Less Than 12 Months, Unrealized Losses 165 29
Available for sale securities, 12 Months or Greater, Market Value      
Available for sale securities, 12 Months or Greater, Unrealized Losses      
ARS [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Available for sale securities, Less Than 12 Months, Market Value      
Available for sale securities, Less Than 12 Months, Unrealized Losses      
Available for sale securities, 12 Months or Greater, Market Value 9,092 8,970
Available for sale securities, 12 Months or Greater, Unrealized Losses $ 1,908 $ 2,030