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Stock-Based Compensation Plans - Stock-Based Compensation Expense (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Employee [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Total stock-based compensation $ 8,869us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_TitleOfIndividualAxis
= mktx_EmployeeCompensationPlanMember		 $ 8,169us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_TitleOfIndividualAxis
= mktx_EmployeeCompensationPlanMember		 $ 7,714us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_TitleOfIndividualAxis
= mktx_EmployeeCompensationPlanMember
Employee [Member] | Restricted Stock and Performance Shares [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Total stock-based compensation 8,193us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= mktx_RestrictedStockAndPerformanceSharesMember
/ us-gaap_TitleOfIndividualAxis
= mktx_EmployeeCompensationPlanMember		 7,269us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= mktx_RestrictedStockAndPerformanceSharesMember
/ us-gaap_TitleOfIndividualAxis
= mktx_EmployeeCompensationPlanMember		 7,046us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= mktx_RestrictedStockAndPerformanceSharesMember
/ us-gaap_TitleOfIndividualAxis
= mktx_EmployeeCompensationPlanMember
Employee [Member] | Stock Options [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Total stock-based compensation 676us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_TitleOfIndividualAxis
= mktx_EmployeeCompensationPlanMember		 900us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_TitleOfIndividualAxis
= mktx_EmployeeCompensationPlanMember		 668us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_TitleOfIndividualAxis
= mktx_EmployeeCompensationPlanMember
Non-Employee Directors [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Total stock-based compensation 9,769us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_TitleOfIndividualAxis
= mktx_NonEmployeeDirectorCompensationPlanMember		 8,879us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_TitleOfIndividualAxis
= mktx_NonEmployeeDirectorCompensationPlanMember		 8,385us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_TitleOfIndividualAxis
= mktx_NonEmployeeDirectorCompensationPlanMember
Non-Employee Directors [Member] | Restricted Stock [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Total stock-based compensation $ 900us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_TitleOfIndividualAxis
= mktx_NonEmployeeDirectorCompensationPlanMember		 $ 710us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_TitleOfIndividualAxis
= mktx_NonEmployeeDirectorCompensationPlanMember		 $ 671us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_TitleOfIndividualAxis
= mktx_NonEmployeeDirectorCompensationPlanMember