Regulatory Capital Requirements - Additional Information (Detail) $ in Millions	Sep. 30, 2021 USD ($)
Brokers And Dealers [Line Items]
Securities reserve deposit	$ 50.1
U.S. Subsidiaries
Brokers And Dealers [Line Items]
Aggregate net capital and financial resources in excess of required level	577.3
Aggregate net capital and financial resources, minimum capital requirement	23.2
U.S. Broker-Dealer Subsidiaries
Brokers And Dealers [Line Items]
Aggregate net capital and financial resources in excess of required level	381.4
Aggregate net capital and financial resources, minimum capital requirement	$ 3.4

Brokers and dealers.

No definition available.

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

Amount of net capital in excess of minimum required net capital, calculated under Alternative Standard.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher SEC
-Name Rule 15c3-1
-Number 240
-Section 15c3-1

Name:	srt_AlternativeExcessNetCapital
Namespace Prefix:	srt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Minimum amount of net capital of broker-dealer and its subsidiary, calculated under Alternative Standard.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher SEC
-Name Rule 15c3-1
-Number 240
-Section 15c3-1

Name:	srt_AlternativeNetCapitalRequirement1
Namespace Prefix:	srt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Securities deposited in a special reserve account for the exclusive benefit of customers pursuant to SEC Regulations.

No definition available.

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

Name:	srt_CounterpartyNameAxis=mktx_USSubsidiariesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_CounterpartyNameAxis=mktx_USBrokerDealerSubsidiariesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: