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Available-for-Sale Securities - Additional Information (Detail) (USD $)
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]    
Other-than-temporary impairment losses $ 0us-gaap_ImpairmentOfInvestments $ 0us-gaap_ImpairmentOfInvestments
Gross realized gains from the sale or maturity of available-for-sale investments 0us-gaap_AvailableForSaleSecuritiesGrossRealizedGains 0us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
Other-than-temporary impairments from the sale or maturity of available-for-sale investments $ 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecurities $ 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecurities