Fair Value Measurements - Summary of Company's Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	$ 23,893
Trading securities, Gross unrealized gains	415
Trading securities, Fair value	24,308
Investments, Amortized cost	23,893	$ 26,761
Investments, Gross unrealized gains	415	1,350
Investments, Fair value	24,308	28,111
Corporate Debt [Member]
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	13,721	19,081
Trading securities, Gross unrealized gains	85	141
Trading securities, Fair value	13,806	19,222
Mutual Funds Held In Rabbi Trust [Member]
Schedule Of Available For Sale Securities [Line Items]
Trading securities, Amortized cost	10,172	7,680
Trading securities, Gross unrealized gains	330	1,209
Trading securities, Fair value	$ 10,502	$ 8,889

Marketable securities accumulated gross unrealized gain before tax.

No definition available.

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

- Definition

Marketable securities amortized cost.

+ References

No definition available.

+ Details

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

- Definition

Trading securities accumulated gross unrealized gain before tax.

+ References

No definition available.

+ Details

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

- Definition

Amount of investment in marketable security.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

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

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

+ Details

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

- Definition

Amortized cost of investments in debt securities classified as trading.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=mktx_MutualFundsHeldInRabbiTrustMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: