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Marketable Investments - Summary of Company's Marketable Investments (Detail) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 134,778 $ 145,512
Gross Unrealized Gains 185 407
Gross Unrealized Losses (2,493) (2,073)
Total short and long term 132,470 143,846
State and Municipal Obligations [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 18,478 18,859
Gross Unrealized Gains 16 27
Gross Unrealized Losses (4) (14)
Total short and long term 18,490 18,872
Federal Agency and Corporate Obligations [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 105,300 115,653
Gross Unrealized Gains 169 380
Gross Unrealized Losses (409) (29)
Total short and long term 105,060 116,004
Total Short-Term Available-for-Sale Securities [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 123,778 134,512
Gross Unrealized Gains 185 407
Gross Unrealized Losses (413) (43)
Total short and long term 123,550 134,876
ARS [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 11,000 11,000
Gross Unrealized Gains      
Gross Unrealized Losses (2,080) (2,030)
Total short and long term $ 8,920 $ 8,970