Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2018	Feb. 15, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K/A
Amendment Flag	true
Amendment Description	This Amendment No. 1 on Form 10-K/A (the “Amendment”) amends the Annual Report on Form 10-K of MarketAxess Holdings Inc. for the fiscal year ended December 31, 2018, as filed with the Securities and Exchange Commission on February 20, 2019 (the “Original Filing”). MarketAxess Holdings Inc. is filing this Amendment for the sole purpose of correcting the number of shares of the Company’s common stock outstanding as of February 15, 2019 as disclosed on the cover page of the Original Filing. The number of outstanding shares of the Company’s common stock disclosed in the Original Filing inadvertently included 2,916,082 treasury shares that were issued but were not outstanding. Therefore, the number of shares of the Company’s common stock outstanding as of February 15, 2019 was 37,669,211.Pursuant to Rule 12b-15 under Securities Exchange Act of 1934, as amended, this Form 10-K/A also contains new Rule 13a-14(a)/15d-14(a) Certifications, which are attached hereto. Because no financial statements have been amended by or included in this Form 10-K/A and this Form 10-K/A does not contain or amend any disclosure with respect to Items 307 and 308 of Regulation S-K, paragraphs 3, 4 and 5 of the certifications have been omitted.This Form 10-K/A does not change or update any of the other disclosures contained in the Original Filing, including, without limitation, the total number of outstanding shares of common stock otherwise disclosed within the body of the Original Filing.
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	MKTX
Entity Registrant Name	MARKETAXESS HOLDINGS INC
Entity Central Index Key	0001278021
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		37,669,211
Entity Public Float			$ 7.2