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Stock-Based Compensation (Details 3) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Unrecognized stock-based compensation expense  
Unrecognized stock compensation expense $ 16,164us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
Weighted average remaining period 2 years 10 months 6 days
Common stock options  
Unrecognized stock-based compensation expense  
Unrecognized stock compensation expense 13,821us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= snta_EmployeeAndDirectorsStockOptionsMember
Weighted average remaining period 2 years 9 months 15 days
Restricted stock  
Unrecognized stock-based compensation expense  
Unrecognized stock compensation expense $ 2,343us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
Weighted average remaining period 3 years 2 months 9 days