Accelerated share repurchases, delivery of shares.

No definition available.

Name:	mktx_AcceleratedShareRepurchasesDeliveryOfShares
Namespace Prefix:	mktx_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Initial settlement payment amount.

+ References

No definition available.

+ Details

Name:	mktx_InitialSettlementPaymentAmount
Namespace Prefix:	mktx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage of total shares delivered initially.

+ References

No definition available.

+ Details

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

- Definition

Stock repurchase program additional authorized repurchase amount1.

+ References

No definition available.

+ Details

Name:	mktx_StockRepurchaseProgramAdditionalAuthorizedRepurchaseAmount1
Namespace Prefix:	mktx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Final price paid per share for the purchase of the targeted number of shares, determined by an average market price over a fixed period of time.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 505
-SubTopic 30
-Name Accounting Standards Codification
-Section 25
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481138/505-30-25-5

+ Details

Name:	us-gaap_AcceleratedShareRepurchasesFinalPricePaidPerShare
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The price paid per share to immediately purchase the targeted number of shares on the date of executing the accelerated share repurchase agreement.

+ References

+ Details

Name:	us-gaap_AcceleratedShareRepurchasesInitialPricePaidPerShare
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Amount remaining authorized for purchase of share under share repurchase plan. Includes, but is not limited to, repurchase of stock and unit of ownership.

+ References

No definition available.

+ Details

Name:	us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-SubTopic 10
-Topic 505
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-2

Reference 2: 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(28))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 3: 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(29))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478448/946-505-50-2

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-09(4)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-3

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-03(i)(2))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479886/946-10-S99-3

Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480008/505-10-S99-1

+ Details

Name:	us-gaap_StockRepurchasedDuringPeriodShares
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Equity impact of the value of stock that has been repurchased during the period and has not been retired and is not held in treasury. Some state laws may mandate the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-SubTopic 10
-Topic 505
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-2

Reference 2: 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(28))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 3: 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(29))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Topic 946
-SubTopic 830
-Name Accounting Standards Codification
-Section 55
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479168/946-830-55-11

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 205
-Name Accounting Standards Codification
-Section 45
-Paragraph 4
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478009/946-205-45-4

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478448/946-505-50-2

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-09(4)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-3

Reference 8: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480008/505-10-S99-1

+ Details

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

- Definition

Number of shares that have been repurchased during the period and are being held in treasury.

+ References

+ Details

Name:	us-gaap_TreasuryStockSharesAcquired
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	srt_ShareRepurchaseProgramAxis=mktx_TwoThousandAndTwentyFourRepurchaseProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ShareRepurchaseProgramAxis=mktx_TwoThousandAndTwentyFiveRepurchaseProgramAndTwoThousandAndTwentyFourRepurchaseProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ShareRepurchaseProgramAxis=mktx_AcceleratedShareRepurchaseProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ShareRepurchaseProgramAxis=mktx_TwoThousandAndTwentyFiveRepurchaseProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Share Repurchase Programs - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		2 Months Ended	3 Months Ended
	Dec. 09, 2025	Feb. 04, 2026	Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025	Aug. 31, 2024
Equity Class Of Treasury Stock [Line Items]
Shares repurchase program, value				$ 38,077
2024 Repurchase Program [Member]
Equity Class Of Treasury Stock [Line Items]
Shares repurchase program authorized additional, value						$ 200,000
2025 Repurchase Program and 2024 Repurchase Program [Member]
Equity Class Of Treasury Stock [Line Items]
Shares repurchase program authorized additional, value					$ 400,000
Shares repurchase program, value			$ 60,000
Shares repurchase program, shares			359,782
Accelerated Share Repurchase Program [Member]
Equity Class Of Treasury Stock [Line Items]
Initial Settlement Payment Amount	$ 300,000
Share repurchases (in shares)	1,386,001
Percentage of shares delivered initially	80.00%
Initial price per share	$ 173.16
Delivery of shares (in shares)		359,782
Final price paid per share		$ 171.84
2025 Repurchase Program
Equity Class Of Treasury Stock [Line Items]
Share Repurchase program, remaining capacity			$ 205,000