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Stock-Based Awards - Summary of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Detail) (USD $)
In Thousands, unless otherwise specified
9 Months Ended 12 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Total stock-based compensation expense $ 910us-gaap_AllocatedShareBasedCompensationExpense $ 252us-gaap_AllocatedShareBasedCompensationExpense $ 395us-gaap_AllocatedShareBasedCompensationExpense $ 121us-gaap_AllocatedShareBasedCompensationExpense $ 80us-gaap_AllocatedShareBasedCompensationExpense
Research and Development [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Total stock-based compensation expense 259us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_ResearchAndDevelopmentExpenseMember
109us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_ResearchAndDevelopmentExpenseMember
176us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_ResearchAndDevelopmentExpenseMember
68us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_ResearchAndDevelopmentExpenseMember
30us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_ResearchAndDevelopmentExpenseMember
General and Administrative [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Total stock-based compensation expense $ 651us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_GeneralAndAdministrativeExpenseMember
$ 143us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_GeneralAndAdministrativeExpenseMember
$ 219us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_GeneralAndAdministrativeExpenseMember
$ 53us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_GeneralAndAdministrativeExpenseMember
$ 50us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_GeneralAndAdministrativeExpenseMember