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Short-term and Long-term Investments - Summary of Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Short-term Investments (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 91,441us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis $ 115,566us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Gross Unrealized Gains 69us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax 54us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Gross Unrealized Losses 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax (12)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Aggregate Fair Value 91,510us-gaap_AvailableForSaleSecuritiesDebtSecurities 115,608us-gaap_AvailableForSaleSecuritiesDebtSecurities
U.S. government debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 91,169us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentDebtSecuritiesMember
115,293us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentDebtSecuritiesMember
Gross Unrealized Gains 69us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentDebtSecuritiesMember
54us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentDebtSecuritiesMember
Gross Unrealized Losses 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentDebtSecuritiesMember
(12)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentDebtSecuritiesMember
Aggregate Fair Value 91,238us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentDebtSecuritiesMember
115,335us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_USGovernmentDebtSecuritiesMember
Certificates of deposit    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 272us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= us-gaap_CertificatesOfDepositMember
273us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= us-gaap_CertificatesOfDepositMember
Gross Unrealized Gains 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= us-gaap_CertificatesOfDepositMember
0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= us-gaap_CertificatesOfDepositMember
Gross Unrealized Losses 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= us-gaap_CertificatesOfDepositMember
0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= us-gaap_CertificatesOfDepositMember
Aggregate Fair Value $ 272us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_CertificatesOfDepositMember
$ 273us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_CertificatesOfDepositMember