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Reconciliation of Federal Statutory Income Tax Rate (Detail)
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Income Tax Disclosure [Abstract]      
Federal statutory income tax rate (34.00%)us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate (34.00%)us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate (34.00%)us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Federal and state research and development tax credit (9.00%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsResearch (1.20%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsResearch (6.40%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsResearch
Orphan drug tax credit (3.10%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxSettlementsOther    
State taxes, net of federal benefit (4.00%)us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes (5.00%)us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes (5.40%)us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes
Meals and entertainment     0.10%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseMealsAndEntertainment
Stock-based compensation expense 0.60%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseShareBasedCompensationCost 0.20%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseShareBasedCompensationCost 0.20%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseShareBasedCompensationCost
Nondeductible Australia research and development expenses 4.10%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseResearchAndDevelopment 1.80%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseResearchAndDevelopment  
Change in deferred tax asset valuation allowance 45.40%us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance 38.20%us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance 45.50%us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance
Effective income tax rate 0.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations 0.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations 0.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations