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Cash, Cash Equivalents and Marketable Securities (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Cash, Cash Equivalents and Marketable Securities    
Cash, cash equivalents and marketable securities, unrealized gains 11snta_CashAndCashEquivalentsAndAvailableForSaleSecuritiesGrossUnrealizedGains 12snta_CashAndCashEquivalentsAndAvailableForSaleSecuritiesGrossUnrealizedGains
Cash, cash equivalents and marketable securities, unrealized losses (5)snta_CashAndCashEquivalentsAndAvailableForSaleSecuritiesGrossUnrealizedLosses (8)snta_CashAndCashEquivalentsAndAvailableForSaleSecuritiesGrossUnrealizedLosses
Cost    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 33,255us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
46,024us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Total cash, cash equivalents and marketable securities 76,600us-gaap_CashCashEquivalentsAndShortTermInvestments
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
97,686us-gaap_CashCashEquivalentsAndShortTermInvestments
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Fair value    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 33,255us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
46,024us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Total cash, cash equivalents and marketable securities 76,606us-gaap_CashCashEquivalentsAndShortTermInvestments
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
97,690us-gaap_CashCashEquivalentsAndShortTermInvestments
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Corporate debt securities due within 1 year of date of purchase    
Cash, Cash Equivalents and Marketable Securities    
Marketable securities, Unrealized gains 11us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
12us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Marketable securities, Unrealized losses (5)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
(8)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Corporate debt securities due within 1 year of date of purchase | Cost    
Cash, Cash Equivalents and Marketable Securities    
Marketable securities 43,345us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
51,662us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Corporate debt securities due within 1 year of date of purchase | Fair value | Level 2    
Cash, Cash Equivalents and Marketable Securities    
Marketable securities 43,351us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
51,666us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Cash and money market funds | Cost    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 21,720us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_CashAndCashEquivalentsAxis
= snta_CashAndMoneyMarketFundsMember
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
45,004us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_CashAndCashEquivalentsAxis
= snta_CashAndMoneyMarketFundsMember
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Cash and money market funds | Fair value | Level 1    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 21,720us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_CashAndCashEquivalentsAxis
= snta_CashAndMoneyMarketFundsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
45,004us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_CashAndCashEquivalentsAxis
= snta_CashAndMoneyMarketFundsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Corporate debt securities due within 3 months of date of purchase | Cost    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 11,535us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_CashAndCashEquivalentsAxis
= snta_CorporateDebtSecuritiesDueWithinThreeMonthsMember
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
1,020us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_CashAndCashEquivalentsAxis
= snta_CorporateDebtSecuritiesDueWithinThreeMonthsMember
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Corporate debt securities due within 3 months of date of purchase | Fair value | Level 2    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 11,535us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_CashAndCashEquivalentsAxis
= snta_CorporateDebtSecuritiesDueWithinThreeMonthsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
1,020us-gaap_CashAndCashEquivalentsFairValueDisclosure
/ us-gaap_CashAndCashEquivalentsAxis
= snta_CorporateDebtSecuritiesDueWithinThreeMonthsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Maximum | Corporate debt securities due within 1 year of date of purchase    
Cash, Cash Equivalents and Marketable Securities    
Maturity period from date of purchase to classify an investment as marketable securities 1 year 1 year
Maximum | Corporate debt securities due within 3 months of date of purchase    
Cash, Cash Equivalents and Marketable Securities    
Maturity period from date of purchase to classify an investment as cash and cash equivalents 3 months 3 months