No definition available.

Name:	mktx_InterestCoverageRatio
Namespace Prefix:	mktx_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

Leverage ratio.

No definition available.

Name:	mktx_LeverageRatio
Namespace Prefix:	mktx_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

Line of credit facility additional borrowing capacity.

No definition available.

Name:	mktx_LineOfCreditFacilityAdditionalBorrowingCapacity
Namespace Prefix:	mktx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Line of credit facility basis spread on aggregate interest rate option one.

No definition available.

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

Line of credit facility basis spread on aggregate interest rate option two.

No definition available.

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

Line of credit facility basis spread on federal funds effective rate.

No definition available.

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

Line of credit facility basis spread on variable rate.

No definition available.

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

Line of credit facility default amount threshold.

No definition available.

Name:	mktx_LineOfCreditFacilityDefaultAmountThreshold
Namespace Prefix:	mktx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Line of credit facility default percentage.

No definition available.

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

Line of credit facility description of variable rate.

No definition available.

Name:	mktx_LineOfCreditFacilityDescriptionOfVariableRate
Namespace Prefix:	mktx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Percentage of foreign subsidiary interest pledged.

No definition available.

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

The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

No definition available.

Name:	us-gaap_LineOfCreditFacilityCommitmentFeePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

Name:	us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).