Retirement and Deferred Compensation Plans - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016
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	$ 3.1	$ 1.3
Trading gain and compensation expense	$ 0.3	$ 0.2

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