Parent Company Information - Condensed Statements of Financial Condition (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
ASSETS
Cash and cash equivalents	$ 460,858	$ 270,124
Investments, at fair value	28,111	230,477
Accounts receivable	79,577	62,017
Intangible assets, net of accumulated amortization	95,354	60,986
Furniture, equipment, leasehold improvements and capitalized software, net of accumulated depreciation and amortization	85,204	71,795
Operating lease right-of-use assets	75,924	81,399
Prepaid expenses and other assets	29,039	31,271
Total assets	1,331,429	954,930
Liabilities
Accrued employee compensation	62,326	47,365
Income and other tax liabilities	42,750	16,690
Accounts payable, accrued expenses and other liabilities	44,354	22,793
Operating lease liabilities	93,612	97,991
Total liabilities	376,368	184,839
Stockholders' equity
Preferred stock
Common stock	123	122
Additional paid-in capital	329,742	342,541
Treasury stock	(169,523)	(153,388)
Retained earnings	799,369	591,086
Accumulated other comprehensive loss	(4,650)	(10,270)
Total stockholders' equity	955,061	770,091	$ 607,878	$ 514,768
Total liabilities and stockholders' equity	1,331,429	954,930
MarketAxess Holdings Inc. (Parent) [Member]
ASSETS
Cash and cash equivalents	55,747	48,241
Investments, at fair value	4,811	141,318
Accounts receivable	178	63
Receivable from subsidiaries	41,986	5,997
Intangible assets, net of accumulated amortization	27	30
Furniture, equipment, leasehold improvements and capitalized software, net of accumulated depreciation and amortization	23,518	25,247
Operating lease right-of-use assets	64,460	68,559
Investments in subsidiaries	853,626	570,145
Prepaid expenses and other assets	4,591	6,978
Income and other tax receivable	9,028	3,219
Total assets	1,057,972	869,797
Liabilities
Accrued employee compensation	10,241	6,543
Income and other tax liabilities	3,457
Accounts payable, accrued expenses and other liabilities	9,341	9,254
Operating lease liabilities	79,872	83,909
Total liabilities	102,911	99,706
Stockholders' equity
Preferred stock
Common stock	123	122
Additional paid-in capital	329,742	342,541
Treasury stock	(169,523)	(153,388)
Retained earnings	799,369	591,086
Accumulated other comprehensive loss	(4,650)	(10,270)
Total stockholders' equity	955,061	770,091
Total liabilities and stockholders' equity	1,057,972	869,797
Series A Preferred Stock [Member]
Stockholders' equity
Preferred stock
Series A Preferred Stock [Member] | MarketAxess Holdings Inc. (Parent) [Member]
Stockholders' equity
Preferred stock