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Short-Term and Long-Term Investments - Additional Information (Detail) - USD ($)
 6 Months Ended
Jun. 30, 2015
Dec. 31, 2014
Jun. 30, 2014
Dec. 31, 2013
Unrealized Losses On Short Term And Long Term Investments [Line Items]        
Amortized cost of securities $ 3,340,000 $ 3,700,000    
Minimum period for which failed auction securities been in unrealized loss position 12 months      
Estimated Fair Value $ 2,955,000 3,272,000    
Gross unrealized losses 386,000 428,000    
Aggregate credit loss 76,000 84,000 $ 375,000 $ 395,000
Failed Auction Security [Member]        
Unrealized Losses On Short Term And Long Term Investments [Line Items]        
Amortized cost of securities $ 3,000,000 3,000,000    
Period for which failed auction securities been in unrealized loss position Exceeds 12 months      
Estimated Fair Value $ 2,614,000 2,575,000    
Gross unrealized losses 386,000 $ 425,000    
Aggregate credit loss 76,000      
Aggregate temporary impairment loss $ 310,000