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Consolidated Statement of Redeemable Preferred Stock and Stockholders' Equity (Deficit) (USD $)
In Thousands, except Share data
Total
USD ($)
Series D Convertible Preferred Stock [Member]
USD ($)
IPO [Member]
USD ($)
Redeemable Convertible Preferred Stock [Member]
USD ($)
Redeemable Convertible Preferred Stock [Member]
Series D Convertible Preferred Stock [Member]
USD ($)
Common Stock [Member]
USD ($)
Common Stock [Member]
IPO [Member]
Additional Paid-in Capital [Member]
USD ($)
Additional Paid-in Capital [Member]
IPO [Member]
USD ($)
Accumulated Deficit [Member]
USD ($)
Redeemable convertible preferred stock, Beginning balance at Dec. 31, 2011       $ 70,751us-gaap_TemporaryEquityValueExcludingAdditionalPaidInCapital
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RedeemableConvertiblePreferredStockMember		            
Beginning balance at Dec. 31, 2011 (58,397)us-gaap_StockholdersEquity             798us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember		   (59,195)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
Redeemable convertible preferred stock, Beginning balance, shares at Dec. 31, 2011       71,374,896us-gaap_TemporaryEquitySharesIssued
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RedeemableConvertiblePreferredStockMember		            
Beginning balance, shares at Dec. 31, 2011           609,041us-gaap_SharesOutstanding
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		        
Exercise of stock options 3us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised             3us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember		    
Exercise of stock options, shares           904us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		        
Sale of convertible preferred stock         12,927us-gaap_TemporaryEquityStockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesDPreferredStockMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RedeemableConvertiblePreferredStockMember		          
Sale of convertible preferred stock, shares         15,662,650ceru_TemporaryEquityStockIssuedDuringPeriodSharesNewIssues
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesDPreferredStockMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RedeemableConvertiblePreferredStockMember		          
Stock-based compensation 529us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue             529us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember		    
Accretion of issuance costs to redemption value (73)us-gaap_TemporaryEquityAccretionToRedemptionValue     73us-gaap_TemporaryEquityAccretionToRedemptionValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RedeemableConvertiblePreferredStockMember		       (73)us-gaap_TemporaryEquityAccretionToRedemptionValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember		    
Net loss (22,101)us-gaap_NetIncomeLoss                 (22,101)us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
Redeemable convertible preferred stock, Ending balance at Dec. 31, 2012       83,751us-gaap_TemporaryEquityValueExcludingAdditionalPaidInCapital
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RedeemableConvertiblePreferredStockMember		            
Ending balance at Dec. 31, 2012 (80,039)us-gaap_StockholdersEquity             1,257us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember		   (81,296)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
Redeemable convertible preferred stock, Ending balance, shares at Dec. 31, 2012       87,037,546us-gaap_TemporaryEquitySharesIssued
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RedeemableConvertiblePreferredStockMember		            
Ending balance, shares at Dec. 31, 2012           609,945us-gaap_SharesOutstanding
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		        
Exercise of stock options 35us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised             35us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember		    
Exercise of stock options, shares           8,911us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		        
Stock-based compensation 622us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue             622us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember		    
Conversion of convertible preferred stock into common stock 2,226us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities     (2,226)us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RedeemableConvertiblePreferredStockMember		       2,226us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember		    
Conversion of convertible preferred stock into common stock, shares       (1,830,190)us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RedeemableConvertiblePreferredStockMember		   166,675us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		        
Net loss (17,143)us-gaap_NetIncomeLoss                 (17,143)us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
Redeemable convertible preferred stock, Ending balance at Dec. 31, 2013 81,525us-gaap_TemporaryEquityValueExcludingAdditionalPaidInCapital 27,521us-gaap_TemporaryEquityValueExcludingAdditionalPaidInCapital
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesDPreferredStockMember		   81,525us-gaap_TemporaryEquityValueExcludingAdditionalPaidInCapital
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RedeemableConvertiblePreferredStockMember		            
Ending balance at Dec. 31, 2013 (94,299)us-gaap_StockholdersEquity             4,140us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember		   (98,439)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
Redeemable convertible preferred stock, Ending balance, shares at Dec. 31, 2013 85,207,356us-gaap_TemporaryEquitySharesIssued 33,158,272us-gaap_TemporaryEquitySharesIssued
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesDPreferredStockMember		   85,207,356us-gaap_TemporaryEquitySharesIssued
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RedeemableConvertiblePreferredStockMember		            
Ending balance, shares at Dec. 31, 2013           785,531us-gaap_SharesOutstanding
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		        
Exercise of stock options 140us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised             140us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember		    
Exercise of stock options, shares 41,566us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised         41,566us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		        
Stock-based compensation 885us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue             885us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember		    
Issuance of Common Stock from initial public offering 59,862us-gaap_StockIssuedDuringPeriodValueNewIssues         1us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		   59,861us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember		    
Issuance of Common Stock from initial public offering, shares             9,569,715us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
/ us-gaap_SubsidiarySaleOfStockAxis
= us-gaap_IPOMember		      
Conversion of convertible preferred stock into common stock 81,526us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities     (81,525)us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RedeemableConvertiblePreferredStockMember		   1us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		   81,525us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember		    
Conversion of convertible preferred stock into common stock, shares       (85,207,356)us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RedeemableConvertiblePreferredStockMember		   6,826,004us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		        
Reclassification of warrants     424us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued
/ us-gaap_SubsidiarySaleOfStockAxis
= us-gaap_IPOMember		           424us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
/ us-gaap_SubsidiarySaleOfStockAxis
= us-gaap_IPOMember		  
Conversion of convertible notes 20,129ceru_ConversionOfConvertibleNotes             20,129ceru_ConversionOfConvertibleNotes
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember		    
Conversion of convertible notes, shares           2,902,233ceru_ConversionOfConvertibleNotesShares
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		        
Net loss (23,342)us-gaap_NetIncomeLoss                 (23,342)us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
Ending balance at Dec. 31, 2014 $ 45,325us-gaap_StockholdersEquity         $ 2us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		   $ 167,104us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember		   $ (121,781)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
Ending balance, shares at Dec. 31, 2014           20,125,049us-gaap_SharesOutstanding
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember