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Stockholders' Equity (Details2) (USD $)
In Millions, except Share data, unless otherwise specified
1 Months Ended 3 Months Ended
Jul. 31, 2014
Mar. 31, 2015
Sep. 30, 2014
Common Stock      
Number of votes per share that common stockholders are entitled to receive   1snta_CommonStockVotingRightsNumber  
At-The-Market Issuance Sales Agreement      
Common Stock Issued      
Maximum aggregate offering price $ 50.0snta_CommonStockAggregateOfferingPrice
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$ 27.0snta_CommonStockAggregateOfferingPrice
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Number of shares sold by the entity     5,679,685us-gaap_StockIssuedDuringPeriodSharesNewIssues
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Value of shares sold by the entity     23.0us-gaap_StockIssuedDuringPeriodValueNewIssues
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Weighted average selling price (in dollars per share)     $ 4.05snta_WeightedAverageSellingPricePerShare
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Net proceeds after deducting commissions and other transactions costs     $ 22.5us-gaap_ProceedsFromIssuanceOrSaleOfEquity
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MLV & Co. LLC | Maximum | At-The-Market Issuance Sales Agreement      
Common Stock Issued      
Percentage of gross proceeds payable as commission   3.00%snta_PercentageOfGrossProceedsPayableAsCommission
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