Stock-Based Compensation Plans - Summary of Performance of Share Activity (Parenthetical) (Detail)	12 Months Ended
Dec. 31, 2011
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock forfeitures	14,563

Share based compensation arrangement by share based payment award equity instruments other than options performance shares net of forfeitures.

No definition available.

Name:	mktx_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceSharesNetOfForfeitures
Namespace Prefix:	mktx_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration