Document and Entity Information - shares
9 Months Ended
Sep. 30, 2019
Oct. 21, 2019
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Sep. 30, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q3  
Trading Symbol MKTX  
Entity Central Index Key 0001278021  
Current Fiscal Year End Date --12-31  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Common Stock, Shares Outstanding   37,777,078
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Shell Company false  
Title of 12(b) Security Common Stock, $0.003 par value  
Security Exchange Name NASDAQ  
Entity File Number 001-34091  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 52-2230784  
Entity Address, Address Line One 55 Hudson Yards, 15th Floor  
Entity Address, City or Town New York  
Entity Address, State or Province NY  
Entity Address, Postal Zip Code 10001  
Document Quarterly Report true  
Document Transition Report false  
City Area Code 212  
Local Phone Number 813-6000