Closing price of common stock.

No definition available.

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- Definition

Number of stock incentive plans.

+ References

No definition available.

+ Details

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- Definition

Share based compensation arrangement by share based payment award equity instruments other than options performance shares payout achievement.

+ References

No definition available.

+ Details

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- Definition

Share based compensation arrangement by share based payment award equity instruments other than options performance shares payout range maximum.

+ References

No definition available.

+ Details

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- Definition

Share based compensation arrangement by share based payment award equity instruments other than options performance shares payout range minimum.

+ References

No definition available.

+ Details

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- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)(2)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (d)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Details

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Stock-Based Compensation Plans - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 Incentive	Dec. 31, 2013	Dec. 31, 2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of stock incentive plans	2mktx_NumberOfStockIncentivePlans
Shares available for grant under the stock incentive plans	988,059
Weighted-average fair value for option granted	$ 19.25	$ 19.61	$ 12.26
Closing price of common stock	$ 71.71mktx_ClosingPriceOfCommonStock
Unrecognized compensation costs related to non-vested	$ 0.4
Weighted-average period over which cost is expected to be recognized	1 year
Pay-out ranges, Minimum	0.00%
Pay-out ranges, Maximum	150.00%
Pay-out achievement	85.00%	108.00%	92.50%
Restricted Stock and Performance Shares [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation costs related to non-vested	$ 11.4
Weighted-average period over which cost is expected to be recognized	1 year 6 months
Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Period vested for option grants	3 years
Options Expiration Period	7 years
Minimum [Member] \| Restricted Stock and Performance Shares [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Period vested for option grants	3 years
Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Period vested for option grants	5 years
Options Expiration Period	10 years
Maximum [Member] \| Restricted Stock and Performance Shares [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Period vested for option grants	5 years