Line of credit facility additional borrowing capacity.

No definition available.

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

- Definition

Maximum available borrowings to subsidiary under agreement.

+ References

No definition available.

+ Details

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

- Definition

Number of additional option to extend maturity date.

+ References

No definition available.

+ Details

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

- Definition

Sublimit for letter of credit.

+ References

No definition available.

+ Details

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

- Definition

Carrying value as of the balance sheet date of payments made in excess of existing cash balances, which will be honored by the bank but reflected as a loan to the entity. Overdrafts generally have a very short time frame for correction or repayment and are therefore more similar to short-term bank financing than trade financing.

+ References

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

+ References

+ Details

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

- Definition

Amount of the cost of borrowed funds accounted for as interest expense for debt.

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The aggregate interest expense incurred on short-term borrowings including commercial paper and Federal funds purchased and securities sold under agreements to repurchase.

+ References

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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+ Details

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Namespace Prefix:	us-gaap_
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

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

- Definition

Description of the terms of a credit facility arrangement. Terms typically include interest rate, collateral required, guarantees required, repayment requirements, and restrictions on use of assets and activities of the entity.

+ References

+ Details

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

- Definition

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

+ References

+ Details

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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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
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

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

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

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

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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

- Details

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- Details

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- Details

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Credit Agreements and Short-term Financing - Additional Information (Detail)			1 Months Ended	9 Months Ended
	Oct. 15, 2021 USD ($) Extension_Option	Nov. 13, 2020 USD ($)	Oct. 31, 2015 USD ($)	Sep. 30, 2021 USD ($)
Line Of Credit Facility [Line Items]
Letter of credit outstanding				$ 1,000,000.0
Interest expense on short-term debt				700,000
Outstanding overdrafts payable				0
Collateralized Agreement [Member]
Line Of Credit Facility [Line Items]
Maximum available borrowings to subsidiary under agreement				200,000,000.0
Outstanding borrowings under agreement				0
Unused borrowing capacity, amount under agreement				200,000,000.0
Collateralized Agreement [Member] \| Minimum [Member]
Line Of Credit Facility [Line Items]
Interest expense on borrowings				$ 100,000
Collateralized Agreement [Member] \| Base Rate [Member]
Line Of Credit Facility [Line Items]
Interest rate, stated percentage				1.00%
Prior Credit Agreement [Member]
Line Of Credit Facility [Line Items]
Revolving loans and letters of credit			$ 100,000,000.0
Expiration period of credit agreement			Nov. 13, 2020
Period of credit agreement		1 year
Revolving Credit Facility [Member]
Line Of Credit Facility [Line Items]
Revolving loans and letters of credit		$ 500,000,000.0
Period of credit agreement				3 years
Amount available under credit agreement				$ 499,000,000.0
Additional borrowings under credit agreement				250,000,000.0
Interest expense on borrowings				$ 0
Line of Credit Facility, Description				On October 15, 2021, the Company refinanced its credit facility to replace the 2020 Credit Agreement with a new three-year revolving credit facility (the “2021 Credit Agreement”) provided by a syndicate of lenders and JPMorgan Chase Bank, N.A., as administrative agent.
Revolving Credit Facility [Member] \| Subsequent Event [Member]
Line Of Credit Facility [Line Items]
Revolving loans and letters of credit	$ 500,000,000.0
Period of credit agreement	364 days
Number of additional option to extend maturity date \| Extension_Option	2
Additional borrowings under credit agreement	$ 250,000,000.0
Standby Letters of Credit [Member]
Line Of Credit Facility [Line Items]
Sub-limit for letter of credit		$ 5,000,000.0
Standby Letters of Credit [Member] \| Subsequent Event [Member]
Line Of Credit Facility [Line Items]
Sub-limit for letter of credit	$ 5,000,000.0