Retirement and Deferred Compensation Plans - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Compensation And Retirement Disclosure [Abstract]
Non-qualified deferred cash incentive plan maximum eligibility percentage of employees	100.00%
Mutual fund investments and deferred compensation obligation, at fair value	$ 4.5	$ 3.1
Trading gains or losses and changes in deferred compensation liability and expense	$ 0.1	$ 0.3

Deferred compensation arrangement trading gains or losses and changes in liability and expense.

No definition available.

Name:	mktx_DeferredCompensationArrangementTradingGainsOrLossesAndChangesInLiabilityAndExpense
Namespace Prefix:	mktx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration