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Fair Value Hierarchy for Financial Assets Measured at Fair Value on Recurring Basis (Detail) (Fair Value, Measurements, Recurring, USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Assets $ 120,130us-gaap_AssetsFairValueDisclosureRecurring $ 187,610us-gaap_AssetsFairValueDisclosureRecurring
Fair Value, Inputs, Level 1
   
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Assets 46,989us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
20,013us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
Fair Value, Inputs, Level 2
   
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Assets 73,141us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
167,597us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
Money Market Funds
   
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Assets 46,989us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
20,013us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
Money Market Funds | Fair Value, Inputs, Level 1
   
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Assets 46,989us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
20,013us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
U.S. Government Agency Securities
   
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Assets 73,141us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
167,597us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
U.S. Government Agency Securities | Fair Value, Inputs, Level 2
   
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Assets $ 73,141us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
$ 167,597us-gaap_AssetsFairValueDisclosureRecurring
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember