Commitments and Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015 Subsidiary Agreement	Mar. 31, 2014
Loss Contingencies [Line Items]
Rental expense	$ 0.9us-gaap_OperatingLeasesRentExpenseNet	$ 0.9us-gaap_OperatingLeasesRentExpenseNet
Number of lease agreements assigned to third parties	2mktx_NumberOfLeaseAgreements
Lease termination dates	November 2015 and November 2020
Future lease obligations under sublease arrangements	1.7us-gaap_ContractualObligation
Number of subsidiaries	2mktx_NumberOfSubsidiaries
Settlement days of bond transaction	Within one to three trading days
Revenues from riskless principal transactions	3.5us-gaap_PrincipalTransactionsRevenue	1.1us-gaap_PrincipalTransactionsRevenue
Collateral deposit	0.9us-gaap_DepositsAssets
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Contingent obligation for standby letter of credit issued to Landlord	$ 1.4

Lease expiration dates.

No definition available.

Name:	mktx_LeaseExpirationDates
Namespace Prefix:	mktx_
Data Type:	us-types:dateStringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of lease agreements.

+ References

No definition available.

+ Details

Name:	mktx_NumberOfLeaseAgreements
Namespace Prefix:	mktx_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of subsidiaries.

+ References

No definition available.

+ Details

Name:	mktx_NumberOfSubsidiaries
Namespace Prefix:	mktx_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Settlement period of bond transaction.

+ References

No definition available.

+ Details

Name:	mktx_SettlementPeriodOfBondTransaction
Namespace Prefix:	mktx_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of contractual obligation, including but not limited to, long-term debt, capital lease obligations, operating lease obligations, purchase obligations, and other commitments.

+ References

No definition available.

+ Details

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

- Definition

Carrying value of amounts transferred to third parties for security purposes that are expected to be returned or applied towards payment in the future.

+ References

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 210.5-02.17)

-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 17

-Article 5

+ Details

Name:	us-gaap_DepositsAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_LossContingenciesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41499-112717

+ Details

Name:	us-gaap_OperatingLeasesRentExpenseNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of realized and unrealized gain (loss) resulting from the difference between acquisition price and selling price or fair value of trading assets and trading liabilities, and from the firm's direct investment activity, conducted separately from customer trading activities, including, but not limited to, investments in private equity, alternative investment products, real estate, and exchanges and memberships.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 940

-SubTopic 320

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=35711157&loc=d3e42567-110969

+ Details

Name:	us-gaap_PrincipalTransactionsRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=us-gaap_StandbyLettersOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: