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Stock-Based Compensation - Additional Information (Detail) (USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jun. 12, 2014
Jun. 12, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Aggregate number of shares of common stock reserved for future issuance 17,100,000us-gaap_CommonStockCapitalSharesReservedForFutureIssuance        
Maximum number of shares of common stock authorized for issuance 225,000,000us-gaap_CommonStockSharesAuthorized 112,500,000us-gaap_CommonStockSharesAuthorized   112,500,000us-gaap_CommonStockSharesAuthorized  
Outstanding options and other stock based awards 11,300,000cers_OptionsAndOtherStockBasedAwards        
Total intrinsic value of options exercised $ 3.8us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue $ 0.6us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue $ 0.3us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue    
Stock-based compensation, expected dividend yield 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate        
Weighted average grant-date fair value of stock options granted $ 3.28us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue $ 2.03us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue $ 2.13us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue    
Stock Options          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Stock-based compensation expense expected to be recognized 9.1us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
       
Stock-based compensation, weighted average recognition period 2 years 4 months 24 days        
Stock-based compensation, expected dividend yield 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
   
Restricted Stock Units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted stock units vesting Description The restricted stock units generally vested in three annual installments from the date of grant and were generally issuable at the end of the three-year vesting term.        
Fair value of restricted stock units which vested during the year   0.05us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
0.05us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
   
Employee Stock Purchase Plan          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Increase in shares of common stock authorized for issuance 1,320,500us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized
/ us-gaap_PlanNameAxis
= cers_EmployeeStockPurchasePlanMember
       
Aggregate number of shares of common stock reserved for future issuance 387,349us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
/ us-gaap_PlanNameAxis
= cers_EmployeeStockPurchasePlanMember
       
Employee Stock Purchase Plan, offering period 12 months        
Employee Stock Purchase Plan, discount on purchase price of common stock as percentage of fair market value 15.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPricePurchaseDate
/ us-gaap_PlanNameAxis
= cers_EmployeeStockPurchasePlanMember
       
Number of purchase periods within each offering period 2cers_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfPurchasePeriod
/ us-gaap_PlanNameAxis
= cers_EmployeeStockPurchasePlanMember
       
Weighted average grant-date fair value of awards granted $ 1.42us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_PlanNameAxis
= cers_EmployeeStockPurchasePlanMember
$ 1.18us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_PlanNameAxis
= cers_EmployeeStockPurchasePlanMember
$ 1.43us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_PlanNameAxis
= cers_EmployeeStockPurchasePlanMember
   
2008 Equity Incentive Plan          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Aggregate number of shares of common stock reserved for future issuance 5,800,000us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
/ us-gaap_PlanNameAxis
= cers_TwoThousandEightEquityIncentivePlanMember
       
Maximum number of shares of common stock authorized for issuance         19,540,940us-gaap_CommonStockSharesAuthorized
/ us-gaap_PlanNameAxis
= cers_TwoThousandEightEquityIncentivePlanMember
Stock-based compensation, award term 10 years        
Stock-based compensation, option to be granted at percentage of fair value of common stock 100.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent
/ us-gaap_PlanNameAxis
= cers_TwoThousandEightEquityIncentivePlanMember
       
Performance-based stock options, outstanding 0cers_PerformanceBasedStockOptionsOutstanding
/ us-gaap_PlanNameAxis
= cers_TwoThousandEightEquityIncentivePlanMember
       
2008 Equity Incentive Plan | Stock Options          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Stock-based compensation, vesting period 4 years        
2008 Equity Incentive Plan | Performance-based Stock or Cash Awards          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Employee Stock Purchase Plan, authorized shares for issuance 500,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_AwardTypeAxis
= cers_PerformanceUnitsAndCashBasedAwardsMember
/ us-gaap_PlanNameAxis
= cers_TwoThousandEightEquityIncentivePlanMember
       
Stock option plan granted on cash award $ 1.0us-gaap_EmployeeServiceShareBasedCompensationCashFlowEffectCashUsedToSettleAwards
/ us-gaap_AwardTypeAxis
= cers_PerformanceUnitsAndCashBasedAwardsMember
/ us-gaap_PlanNameAxis
= cers_TwoThousandEightEquityIncentivePlanMember