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Contractual Maturities of Debt Securities (Detail) (USD $)
Jun. 30, 2013
Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]    
Due in 1 year or less $ 20,805,600  
Due in 1 to 2 years 4,646,013  
Available-for-sale Debt Securities, Amortized Cost Basis, Total 25,451,613  
Due in 1 year or less 20,813,833  
Due in 1 to 2 years 4,645,288  
Fair Value $ 25,459,121 $ 20,760,050