Changes of debt amount under calculation of consolidated leverage ratio.

No definition available.

Name:	mktx_ChangesOfDebtAmountUnderCalculationOfConsolidatedLeverageRatio
Namespace Prefix:	mktx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Debt instrument, credit spread adjustment.

+ References

No definition available.

+ Details

Name:	mktx_DebtInstrumentCreditSpreadAdjustment
Namespace Prefix:	mktx_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line of credit facility additional borrowing capacity.

+ References

No definition available.

+ Details

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

- Definition

Standby letter of credit.

+ References

No definition available.

+ Details

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

- Definition

Sub-limit for swingline loans.

+ References

No definition available.

+ Details

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

- Definition

Sublimit for letter of credit.

+ References

No definition available.

+ Details

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

- Definition

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.

+ References

+ Details

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

- Definition

Date the credit facility terminates, in YYYY-MM-DD format.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityExpirationDate1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Period remaining on line of credit facility before it terminates, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityExpirationPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 852
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481372/852-10-55-10

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16)(a)(1))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

+ Details

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

- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 830
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481674/830-30-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 855
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483399/855-10-50-2

+ Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=mktx_TwoThousandTwentySixAmendedCreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=mktx_CreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Subsequent Events - Additional Information (Detail) $ in Thousands				12 Months Ended
	Feb. 02, 2029	Feb. 04, 2026 USD ($) LetterOfCredit	Aug. 09, 2023 USD ($)	Dec. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)
Subsequent Event [Line Items]
Borrowings				$ 220,000	$ 0
2026 Amended Credit Agreement [Member] \| Subsequent Event [Member]
Subsequent Event [Line Items]
Sub-limit for swingline loans		$ 380,000
Credit spread adjustment		0.10%
Borrowings		$ 220,000
2026 Amended Credit Agreement [Member] \| Maximum [Member] \| Subsequent Event [Member]
Subsequent Event [Line Items]
Changes of debt amount under the calculation of consolidated leverage ratio		200,000
2026 Amended Credit Agreement [Member] \| Minimum [Member] \| Subsequent Event [Member]
Subsequent Event [Line Items]
Changes of debt amount under the calculation of consolidated leverage ratio		$ 30,000
2023 Credit Agreement [Member]
Subsequent Event [Line Items]
Sub-limit for swingline loans			$ 380,000
2023 Credit Agreement [Member] \| Subsequent Event [Member]
Subsequent Event [Line Items]
Standby letter of credit \| LetterOfCredit		1
Revolving Credit Facility [Member] \| 2026 Amended Credit Agreement [Member] \| Subsequent Event [Member]
Subsequent Event [Line Items]
Revolving loans and letters of credit		$ 750,000
Additional borrowings under credit agreement		$ 375,000
Expiration period of credit agreement	Feb. 02, 2029
Period of credit agreement		364 days
Revolving Credit Facility [Member] \| 2023 Credit Agreement [Member]
Subsequent Event [Line Items]
Revolving loans and letters of credit			$ 750,000
Additional borrowings under credit agreement				$ 375,000
Expiration period of credit agreement			Aug. 09, 2026
Period of credit agreement				364 days
Borrowings				$ 220,000
Standby Letters of Credit [Member] \| 2026 Amended Credit Agreement [Member] \| Subsequent Event [Member]
Subsequent Event [Line Items]
Sub-limit for letter of credit		$ 5,000
Standby Letters of Credit [Member] \| 2023 Credit Agreement [Member]
Subsequent Event [Line Items]
Sub-limit for letter of credit			$ 5,000