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Earnings Per Share - Additional Information (Detail) (Stock Options and Restricted Stock [Member])
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Stock Options and Restricted Stock [Member]
     
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]      
Stock options and restricted stock excluded from the computation of diluted earnings per share 66,637us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AwardTypeAxis
= mktx_StockOptionsAndRestrictedStockMember
160,838us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AwardTypeAxis
= mktx_StockOptionsAndRestrictedStockMember
86,346us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AwardTypeAxis
= mktx_StockOptionsAndRestrictedStockMember