Parent Company Information - Condensed Statements of Operations and Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Expenses
Employee compensation and benefits	$ 156,885	$ 131,079	$ 109,117
Depreciation and amortization	35,996	26,857	23,080
Technology and communications	34,092	26,792	23,866
Professional and consulting fees	32,304	25,534	21,521
Occupancy	13,425	11,639	14,176
General and administrative	12,697	15,696	11,353
Total expenses	314,397	260,470	222,981
Operating (loss) income	374,728	250,882	212,584
Other income (expense)
Investment income	2,446	8,063	6,112
Interest expense	(1,142)
Other, net	(1,673)	(1,521)	(610)
Total other income (expense)	(369)	6,542	5,502
Provision for income taxes	74,982	52,522	45,234
Net income	299,377	204,902	172,852
Comprehensive income	304,997	207,026	170,684
MarketAxess Holdings Inc. (Parent) [Member]
Dividends from subsidiary	30,000	165,000	130,000
Expenses
Employee compensation and benefits	19,710	16,100	10,785
Depreciation and amortization	2,068	1,919	1,760
Technology and communications	10	13	43
Professional and consulting fees	7,332	6,523	4,932
Occupancy	162	850	7,381
General and administrative	2,551	2,252	1,513
Total expenses	31,833	27,657	26,414
Operating (loss) income	(1,833)	137,343	103,586
Other income (expense)
Investment income	2,799	5,305	3,835
Interest expense	(805)
Other, net	(318)	(1,344)	(711)
Total other income (expense)	1,676	3,961	3,124
(Loss) income before income taxes and equity in undistributed earnings of subsidiaries	(157)	141,304	106,710
Provision for income taxes	(23,444)	(9,442)	(7,350)
Income before equity in undistributed income of subsidiaries	23,287	150,746	114,060
Equity in undistributed income of subsidiaries	276,090	54,156	58,792
Net income	299,377	204,902	172,852
Other comprehensive income (loss), net	5,620	2,124	(2,168)
Comprehensive income	$ 304,997	$ 207,026	$ 170,684