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Stock-Based Compensation (Details 2) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Stock-based compensation expense    
Stock-based compensation expense $ 1,710us-gaap_AllocatedShareBasedCompensationExpense $ 2,586us-gaap_AllocatedShareBasedCompensationExpense
Research and development    
Stock-based compensation expense    
Stock-based compensation expense 981us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_ResearchAndDevelopmentExpenseMember		 1,131us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_ResearchAndDevelopmentExpenseMember
General and administrative    
Stock-based compensation expense    
Stock-based compensation expense 729us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_GeneralAndAdministrativeExpenseMember		 1,455us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_GeneralAndAdministrativeExpenseMember
Common stock options    
Stock-based compensation expense    
Stock-based compensation expense 1,508us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= snta_EmployeeAndDirectorsStockOptionsMember		 2,529us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= snta_EmployeeAndDirectorsStockOptionsMember
Restricted stock    
Stock-based compensation expense    
Stock-based compensation expense $ 202us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember		 $ 57us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember