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Stock-Based Compensation Plans - 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 [Line Items]    
Total stock-based compensation $ 3,042us-gaap_AllocatedShareBasedCompensationExpense $ 2,316us-gaap_AllocatedShareBasedCompensationExpense
Employees [Member]    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Total stock-based compensation 2,806us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_TitleOfIndividualAxis
= mktx_EmployeeCompensationPlanMember		 2,089us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_TitleOfIndividualAxis
= mktx_EmployeeCompensationPlanMember
Non-Employee Directors [Member]    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Total stock-based compensation $ 236us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_TitleOfIndividualAxis
= mktx_NonEmployeeDirectorCompensationPlanMember		 $ 227us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_TitleOfIndividualAxis
= mktx_NonEmployeeDirectorCompensationPlanMember