Parent Company Information - Condensed Statements of Operations and Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Expenses
Employee compensation and benefits	$ 131,079	$ 109,117	$ 102,313
Depreciation and amortization	26,857	23,080	19,274
Technology and communications	26,792	23,866	20,048
Professional and consulting fees	25,534	21,521	19,367
Occupancy	11,639	14,176	6,125
General and administrative	15,696	11,353	11,121
Total expenses	260,470	222,981	193,807
Operating income	250,882	212,584	199,615
Other income (expense)
Investment income	8,063	6,112	3,619
Other, net	(1,521)	(610)	(1,466)
Total other income	6,542	5,502	2,153
Provision for income taxes	52,522	45,234	53,679
Net income	204,902	172,852	148,089
Comprehensive income	207,026	170,684	150,091
MarketAxess Holdings Inc. (Parent) [Member]
Dividends from subsidiary	165,000	130,000	120,000
Expenses
Employee compensation and benefits	16,100	10,785	11,289
Depreciation and amortization	1,919	1,760	1,971
Technology and communications	13	43	41
Professional and consulting fees	6,523	4,932	4,185
Occupancy	850	7,381	143
General and administrative	2,252	1,513	852
Total expenses	27,657	26,414	18,481
Operating income	137,343	103,586	101,519
Other income (expense)
Investment income	5,305	3,835	2,608
Other, net	(1,344)	(711)	(685)
Total other income	3,961	3,124	1,923
Income before income taxes and equity in undistributed income of subsidiaries	141,304	106,710	103,442
Provision for income taxes	(9,442)	(7,350)	(13,935)
Income before equity in undistributed income of subsidiaries	150,746	114,060	117,377
Equity in undistributed income of subsidiaries	54,156	58,792	30,712
Net income	204,902	172,852	148,089
Other comprehensive income (loss), net	2,124	(2,168)	2,002
Comprehensive income	$ 207,026	$ 170,684	$ 150,091