Deferred Compensation Plans - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018
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	$ 5.8	$ 4.4
Trading (losses) gains and changes in deferred compensation liability and expense	$ 0.7

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

No definition available.

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