Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024
Cash flows from operating activities
Net income	$ 86,245	$ 137,553
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	37,431	36,556
Amortization of operating lease right-of-use assets	3,626	3,238
Stock-based compensation expense	15,345	15,084
Deferred taxes	(706)	(2,461)
Foreign currency transaction losses	4,080	1,220
Other	(2,826)	509
Changes in operating assets and liabilities:
(Increase) in accounts receivable	(13,146)	(2,882)
(Increase)/decrease in receivables from broker-dealers, clearing organizations and customers	(159,675)	87,335
Decrease/(increase) in prepaid expenses and other assets	2,826	(260)
(Decrease) in accrued employee compensation	(19,696)	(17,843)
Increase/(decrease) in payables to broker-dealers, clearing organizations and customers	99,968	(144,404)
Increase in securities sold, not yet purchased	0	9,167
Increase/(decrease) in income and other tax liabilities	91,406	(5,197)
(Decrease)/increase in accounts payable, accrued expenses and other liabilities	(6,913)	1,106
(Decrease) in operating lease liabilities	(4,373)	(4,278)
Net cash provided by operating activities	133,343	113,900
Cash flows from investing activities
Proceeds from maturities and sales	7,246	7,464
Purchases	(9,231)	(7,866)
Acquisition, net of cash acquired	(36,515)	0
Purchases of furniture, equipment and leasehold improvements	(3,136)	(8,892)
Capitalization of software development costs	(26,541)	(24,459)
Net cash (used in) investing activities	(68,177)	(33,753)
Cash flows from financing activities
Cash dividends on common stock	(57,705)	(57,296)
Exercise of stock options	0	1,977
Withholding tax payments on Full Value Awards vesting and stock option exercises	(9,846)	(15,099)
Repurchases of common stock	(74,764)	(43,597)
Proceeds from short-term borrowings	0	100,000
Repayments of short-term borrowings	0	(100,000)
Net cash (used in) financing activities	(142,315)	(114,015)
Effect of exchange rate changes on cash and cash equivalents	27,144	(3,674)
Cash and cash equivalents including restricted cash
Net decrease for the period	(50,005)	(37,542)
Beginning of period	700,459	611,672
End of period	650,454	574,130
Supplemental cash flow information
Cash paid for income taxes	9,852	47,207
Cash paid for interest	342	857
Non-cash investing and financing activity
Operating lease right-of-use assets obtained in exchange for operating lease liabilities	407	760
Furniture, equipment, software and leasehold improvement additions included in accounts payable	664	485
Stock-based and accrued incentive compensation relating to capitalized software development costs	3,633	2,431
Exercise of stock options - cashless	0	1,735
Fair value of assets acquired	86,413	0
Cash paid for acquisition, net of cash and cash equivalents acquired	(36,515)	0
Fair value of previously held interest on acquisition date	(34,321)	0
Fair value of remaining noncontrolling interests on acquisition date	(13,755)	0
Liabilities assumed	1,822	0
Trading Investments [Member]
Changes in operating assets and liabilities:
(Increase)/decrease in trading investments	(66)	155
Mutual Funds Held In Rabbi Trust [Member]
Changes in operating assets and liabilities:
(Increase)/decrease in trading investments	$ (183)	$ (698)