Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operating activities
Net Income (Loss)	$ 258,055	$ 250,224	$ 257,888
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	70,557	61,446	53,447
Amortization of operating lease right-of-use assets	5,853	5,708	6,799
Stock-based compensation expense	29,190	29,864	27,314
Deferred taxes	(5,815)	(6,547)	3,118
Foreign currency transaction (gains) losses	4,718	(8,783)	0
Other	(3,113)	555	(466)
Changes in operating assets and liabilities:
(Increase) decrease in accounts receivable	(7,116)	(15,136)	15,598
(Increase) in receivables from broker-dealers, clearing organizations and customers	(181,044)	(47,631)	(156,909)
Decrease/(increase) in prepaid expenses and other assets	(11,898)	(4,249)	2,214
Increase/(decrease) in accrued employee compensation	1,466	(3,417)	(2,607)
Increase in payables to broker-dealers, clearing organizations and customers	227,920	74,668	95,999
(Decrease) in income and other tax liabilities	(14,691)	(4,768)	(5,638)
(Decrease)/increase in accounts payable, accrued expenses and other liabilities	(7,229)	11,384	215
(Decrease) in operating lease liabilities	(6,735)	(6,373)	(7,001)
Net cash provided by operating activities	333,767	289,231	282,091
Cash flows from investing activities
Proceeds from maturities and sales	4,452	0	0
Purchases	(28,818)	0	0
Acquisitions, net of cash and cash equivalents acquired	(78,476)	0	(17,078)
Acquisition of equity method investment	0	(34,400)	0
Purchases of furniture, equipment and leasehold improvements	(9,326)	(13,142)	(17,493)
Capitalization of software development costs	(43,122)	(38,730)	(33,123)
Net cash (used in) investing activities	(155,290)	(86,272)	(67,694)
Cash flows from financing activities
Cash dividend on common stock	(109,658)	(105,942)	(99,792)
Exercise of stock options	940	672	7,096
Withholding tax payments on restricted stock vesting and stock option exercises	(25,839)	(23,404)	(33,890)
Repurchases of common stock	0	(87,540)	(63,189)
Payment of contingent consideration	(12,500)	(26,164)	0
Proceeds from short-term borrowings	123,995	100,000	70,348
Repayments of short-term borrowings	(123,995)	(100,000)	(70,348)
Net cash (used in) financing activities	(147,057)	(242,378)	(189,775)
Effect of exchange rate changes on cash and cash equivalents	7,588	(13,484)	(7,105)
Cash and cash equivalents including restricted cash
Net increase (decrease) for the period	39,008	(52,903)	17,517
Beginning of period	572,664	625,567	608,050
End of period	611,672	572,664	625,567
Supplemental cash flow information:
Cash paid for income taxes	94,814	88,677	70,003
Cash paid for interest	1,870	652	830
Non-cash investing and financing activity:
Exercise of stock options - cashless	0	3,845	2,750
Right-of-use assets obtained in exchange for operating lease liabilities	1,183	1,880	1,972
Contingent consideration payable recognized in connection with acquisitions	0	0	27,947
Fair value of assets acquired	127,635	0	0
Cash paid for acquisition, net of cash and cash equivalents acquired	(78,476)	0	0
Treasury shares used for acquisition	(43,841)	0	0
Liabilities assumed	5,318	0	0
Trading Investments [Member]
Changes in operating assets and liabilities:
(Increase) in trading investments	(25,248)	(49,527)	(5,574)
Mutual Funds Held In Rabbi Trust [Member]
Changes in operating assets and liabilities:
(Increase) in trading investments	$ (1,103)	$ 1,813	$ (2,306)