Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Cash flows from operating activities
Net Income (Loss)	$ 274,181	$ 258,055	$ 250,224
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	73,824	70,557	61,446
Amortization of operating lease right-of-use assets	6,639	5,853	5,708
Stock-based compensation expense	29,684	29,190	29,864
Deferred taxes	(3,878)	(5,815)	(6,547)
Foreign currency transaction losses/(gains)	1,276	4,718	(8,783)
Other	7,042	(3,113)	555
Changes in operating assets and liabilities:
(Increase) in accounts receivable	(3,730)	(7,116)	(15,136)
Decrease/(increase) in receivables from broker-dealers, clearing organizations and customers	320,091	(181,044)	(47,631)
Decrease/(increase) in prepaid expenses and other assets	1,009	(11,898)	(4,249)
Increase in accrued employee compensation	7,049	1,466	(3,417)
(Decrease)/increase in payables to broker-dealers, clearing organizations and customers	(318,229)	227,920	74,668
(Decrease) in income and other tax liabilities	(3,284)	(14,691)	(4,768)
Increase/(decrease) in accounts payable, accrued expenses and other liabilities	3,543	(7,229)	11,384
(Decrease) in operating lease liabilities	(8,730)	(6,735)	(6,373)
Net cash provided by operating activities	385,237	333,767	289,231
Cash flows from investing activities
Proceeds from maturities and sales	12,440	4,452	0
Purchases	(42,810)	(28,818)	0
Acquisitions, net of cash and cash equivalents acquired	0	(78,476)	0
Acquisition of equity method investment	0	0	(34,400)
Purchases of furniture, equipment and leasehold improvements	(9,942)	(9,326)	(13,142)
Capitalization of software development costs	(46,623)	(43,122)	(38,730)
Net cash (used in) investing activities	(86,935)	(155,290)	(86,272)
Cash flows from financing activities
Cash dividends on common stock	(112,697)	(109,658)	(105,942)
Exercise of stock options	2,988	940	672
Withholding tax payments on Full Value Awards vesting and stock option exercises	(16,194)	(25,839)	(23,404)
Repurchases of common stock	(75,474)	0	(87,540)
Payment of contingent consideration	0	(12,500)	(26,164)
Proceeds from short-term borrowings	344,925	123,995	100,000
Repayments of short-term borrowings	(344,925)	(123,995)	(100,000)
Net cash (used in) financing activities	(201,377)	(147,057)	(242,378)
Effect of exchange rate changes on cash and cash equivalents	(8,138)	7,588	(13,484)
Cash and cash equivalents including restricted cash
Net increase for the period	88,787	39,008	(52,903)
Beginning of period	611,672	572,664	625,567
End of period	700,459	611,672	572,664
Supplemental cash flow information
Cash paid for income taxes	96,932	94,814	88,677
Cash paid for interest	1,717	1,870	652
Non-cash investing and financing activity
Operating lease right-of-use assets obtained in exchange for operating lease liabilities	1,840	1,183	1,880
Furniture, equipment, software and leasehold improvement additions included in accounts payable	693	0	0
Stock-based and accrued incentive compensation relating to capitalized software development costs	5,542	0	0
Exercise of stock options - cashless	1,735	0	3,845
Fair value of assets acquired	0	127,635	0
Cash paid for acquisition, net of cash and cash equivalents acquired	0	(78,476)	0
Treasury shares used for acquisition	0	(43,841)	0
Liabilities assumed	0	5,318	0
Trading Investments [Member]
Changes in operating assets and liabilities:
Decrease/(increase) in trading investments	(629)	(25,248)	(49,527)
Mutual Funds Held In Rabbi Trust [Member]
Changes in operating assets and liabilities:
Decrease/(increase) in trading investments	$ (621)	$ (1,103)	$ 1,813