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Fair Value Measurements - Summary of Financial Assets and Liabilities Measured on Recurring Basis at Fair Value (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market fund $ 50,541us-gaap_MoneyMarketFundsAtCarryingValue $ 5,233us-gaap_MoneyMarketFundsAtCarryingValue
Preferred stock warrant liability   928us-gaap_WarrantsAndRightsOutstanding
Significant Other Observable Inputs (Level 2) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market fund 50,541us-gaap_MoneyMarketFundsAtCarryingValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
5,233us-gaap_MoneyMarketFundsAtCarryingValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Significant Unobservable Inputs (Level 3) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Preferred stock warrant liability   $ 928us-gaap_WarrantsAndRightsOutstanding
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member