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Income Taxes - Reconciliation of Income Tax Expense Computed at Statutory Federal Income Tax Rate to Income Taxes (Detail)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Income Tax Disclosure [Abstract]    
Federal income tax expense at statutory rate 34.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 34.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
State income tax, net of federal benefit 4.50%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes 5.00%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes
Permanent differences (3.70%)ceru_EffectiveIncomeTaxRateReconciliationPermanentDifferences 0.20%ceru_EffectiveIncomeTaxRateReconciliationPermanentDifferences
Research and development credit 0.80%us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsResearch 1.40%us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsResearch
Stock compensation (0.90%)us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseShareBasedCompensationCost (1.10%)us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseShareBasedCompensationCost
Expiration of state net operating loss (1.70%)ceru_EffectiveIncomeTaxRateReconciliationExpirationOfStateNetOperatingLoss (4.20%)ceru_EffectiveIncomeTaxRateReconciliationExpirationOfStateNetOperatingLoss
Other (0.80%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments 0.20%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
Change in valuation allowance (32.20%)us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance (35.50%)us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance
Effective income tax rate 0.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations 0.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations