Parent Company Information - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net income	$ 299,377	$ 204,902	$ 172,852
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	35,996	26,857	23,080
Amortization of operating lease right-of-use assets	6,842	5,795
Stock-based compensation expense	25,613	25,294	15,850
Deferred taxes	10,099	2,674	(1,173)
Other	(550)	(778)	2,253
Changes in operating assets and liabilities:
(Increase) in accounts receivable	(18,015)	(2,962)	(5,117)
(Increase) in prepaid expenses and other assets	(1,977)	(4,624)	(3,791)
Decrease (Increase) in trading securities	67,952	4,045	856
Increase in accrued employee compensation	14,961	8,312	2,551
Increase in income and other tax liabilities	16,189	187	2,965
Increase (decrease) in accounts payable, accrued expenses and other liabilities	6,006	(820)	14,524
(Decrease) in operating lease liabilities	(5,788)	(829)
Net cash provided by operating activities	404,489	265,935	223,917
Cash flows from investing activities
Acquisition of business, net of cash and cash equivalents acquired		(97,430)
Proceeds from maturities and sales, available-for-sale investments	170,657	170,936	333,791
Purchases, available-for-sale investments	(32,865)	(160,827)	(336,533)
Purchases of furniture, equipment and leasehold improvements	(15,010)	(12,292)	(35,888)
Capitalization of software development costs	(30,618)	(22,408)	(11,705)
Other		(30)	39
Net cash provided by (used in) investing activities	68,867	(122,051)	(50,296)
Cash flows from financing activities
Cash dividend on common stock	(90,566)	(76,231)	(62,432)
Exercise of stock options	4,007	1,207	2,973
Withholding tax payments on restricted stock vesting and stock option exercises	(42,418)	(25,820)	(8,043)
Repurchases of common stock	(16,135)	(17,256)	(25,171)
Proceeds from short-term borrowings	578,356
Repayments of short-term borrowings	(578,356)
Net cash (used in) financing activities	(145,112)	(118,100)	(92,673)
Effect of exchange rate changes on investments	5,553	1,011	(1,640)
Cash and cash equivalents including restricted cash
Net increase for the period	333,797	26,795	79,308
Beginning of period	274,253	247,458	168,150
End of period	608,050	274,253	247,458
Supplemental cash flow information:
Cash paid for income taxes	45,046	51,766	47,208
Cash paid for interest	1,142
Non-cash investing and financing activity:
Exercise of stock options - cashless	10,866	1,811
Treasury stock used for acquisition of business		(48,830)
MarketAxess Holdings Inc. (Parent) [Member]
Cash flows from operating activities
Net income	299,377	204,902	172,852
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	2,068	1,919	1,760
Amortization of operating lease right-of-use assets	4,117	4,027
Stock-based compensation expense	10,834	10,547	5,801
Deferred taxes	3,644	1,255	(2,515)
Equity in undistributed income of subsidiaries	(276,090)	(54,156)	(58,792)
Other	(671)	328	710
Changes in operating assets and liabilities:
(Increase) in accounts receivable	(115)	(4)	(14)
(Increase) decrease in receivable from subsidiaries	(25,049)	5,253	15,985
(Increase) in prepaid expenses and other assets	(1,085)	933	(782)
Increase in accrued employee compensation	3,698	876	567
(Increase) in income and other tax receivable	(1,240)	(3,219)
Increase in income and other tax liabilities	6,676	(2,612)	469
Increase (decrease) in accounts payable, accrued expenses and other liabilities	(442)	(2,039)	13,619
(Decrease) in operating lease liabilities	(4,055)	1,191
Net cash provided by operating activities	20,339	168,018	149,190
Cash flows from investing activities
Acquisition of business, net of cash and cash equivalents acquired		(102,320)
Proceeds from maturities and sales, available-for-sale investments	170,657	170,936	333,791
Purchases, available-for-sale investments	(32,865)	(160,827)	(336,533)
Purchases of furniture, equipment and leasehold improvements	(337)	(1,424)	(24,786)
Purchase of intangible asset		(30)
Net cash provided by (used in) investing activities	137,455	(93,665)	(27,528)
Cash flows from financing activities
Cash dividend on common stock	(90,566)	(76,231)	(62,432)
Exercise of stock options	4,007	1,207	2,973
Withholding tax payments on restricted stock vesting and stock option exercises	(42,418)	(25,820)	(8,043)
Repurchases of common stock	(16,135)	(17,256)	(25,171)
Proceeds from short-term borrowings	348,000
Repayments of short-term borrowings	(348,000)
Net cash (used in) financing activities	(145,112)	(118,100)	(92,673)
Effect of exchange rate changes on investments	(5,176)	(3,852)	4,129
Cash and cash equivalents including restricted cash
Net increase for the period	7,506	(47,599)	33,118
Beginning of period	48,241	95,840	62,722
End of period	55,747	48,241	95,840
Supplemental cash flow information:
Cash paid for income taxes	32,674	41,025	37,721
Cash paid for interest	805
Non-cash investing and financing activity:
Exercise of stock options - cashless	10,866	1,811
Treasury stock used for acquisition of business		(48,830)
Mutual Funds Held In Rabbi Trust [Member]
Changes in operating assets and liabilities:
Decrease (Increase) in trading securities	(2,671)	(2,118)	(933)
Mutual Funds Held In Rabbi Trust [Member] | MarketAxess Holdings Inc. (Parent) [Member]
Changes in operating assets and liabilities:
Decrease (Increase) in trading securities	$ (1,328)	$ (1,183)	$ (470)