Retirement and Deferred Compensation Plans - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 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	$ 5.6	$ 4.3

Non-qualified deferred cash incentive plan maximum eligibility percentage of employees.

No definition available.

Name:	mktx_NonQualifiedDeferredCashIncentivePlanMaximumEligibilityPercentageOfEmployees
Namespace Prefix:	mktx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration