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Stock-Based Compensation - Schedule of Recognized Stock-Based Compensation Expense (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Stock-based compensation expense for employee awards $ 3,344us-gaap_AllocatedShareBasedCompensationExpense $ 2,973us-gaap_AllocatedShareBasedCompensationExpense
Stock-based compensation expense for non-employee awards 26mack_IssuanceOfStockAndWarrantsForServicesOrClaimsExpenseBenefitNonEmployee (37)mack_IssuanceOfStockAndWarrantsForServicesOrClaimsExpenseBenefitNonEmployee
Total stock-based compensation 3,370mack_AllocatedShareBasedCompensationExpenseIncludingNonEmployees 2,936mack_AllocatedShareBasedCompensationExpenseIncludingNonEmployees
Research and Development [Member]    
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Stock-based compensation expense for employee awards 1,967us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_ResearchAndDevelopmentExpenseMember		 1,681us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_ResearchAndDevelopmentExpenseMember
General and Administrative [Member]    
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Stock-based compensation expense for employee awards $ 1,377us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_GeneralAndAdministrativeExpenseMember		 $ 1,292us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_GeneralAndAdministrativeExpenseMember