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Short-term and Long-term investments - Additional Information (Detail) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Amortized Cost and Fair Value Debt Securities [Abstract]      
Unrealized (losses) gains of available-for-sale securities, net of tax $ (378,392)us-gaap_AvailableforsaleSecuritiesGrossUnrealizedGainLoss1 $ 25,467us-gaap_AvailableforsaleSecuritiesGrossUnrealizedGainLoss1 $ 759,564us-gaap_AvailableforsaleSecuritiesGrossUnrealizedGainLoss1
Security considered held-to-maturity $ 0us-gaap_HeldToMaturitySecurities $ 0us-gaap_HeldToMaturitySecurities