Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Cash flows from operating activities
Net Income (Loss)	$ 15,065	$ 72,615
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	18,236	18,200
Amortization of operating lease right-of-use assets	1,763	1,599
Stock-based compensation expense	7,333	7,103
Deferred taxes	(124)	(1,338)
Foreign currency transaction losses	1,691	187
Other	(4,809)	1,730
Changes in operating assets and liabilities:
(Increase) in accounts receivable	(16,414)	(10,491)
(Increase)/decrease in receivables from broker-dealers, clearing organizations and customers	(129,508)	21,735
Decrease/(increase) in prepaid expenses and other assets	5,570	(1,323)
(Decrease) in accrued employee compensation	(34,310)	(29,017)
Increase/(decrease) in payables to broker-dealers, clearing organizations and customers	96,275	(71,135)
Increase/(decrease) in income and other tax liabilities	77,295	(4,001)
(Decrease) in accounts payable, accrued expenses and other liabilities	(7,146)	(8,385)
(Decrease) in operating lease liabilities	(2,125)	(2,154)
Net cash provided by/(used in) operating activities	29,629	(4,949)
Cash flows from investing activities
Proceeds from maturities and sales	5,271	4,302
Purchases	(5,458)	(4,972)
Purchases of furniture, equipment and leasehold improvements	(1,930)	(1,197)
Capitalization of software development costs	(15,031)	(13,963)
Net cash (used in) investing activities	(17,148)	(15,830)
Cash flows from financing activities
Cash dividends on common stock	(29,464)	(29,480)
Exercise of stock options	0	1,977
Withholding tax payments on Full Value Awards vesting and stock option exercises	(9,525)	(14,893)
Repurchases of common stock	(38,077)	(10,147)
Net cash (used in) financing activities	(77,066)	(52,543)
Effect of exchange rate changes on cash and cash equivalents	8,580	(3,186)
Cash and cash equivalents including restricted cash
Net decrease for the period	(56,005)	(76,508)
Beginning of period	700,459	611,672
End of period	644,454	535,164
Supplemental cash flow information
Cash paid/(refunded) for income taxes	(1,874)	37,187
Cash paid for interest	198	431
Non-cash investing and financing activity
Operating lease right-of-use assets obtained in exchange for operating lease liabilities	92	630
Furniture, equipment, software and leasehold improvement additions included in accounts payable	501	4,825
Stock-based and accrued incentive compensation relating to capitalized software development costs	1,754	921
Exercise of stock options - cashless	0	1,735
Trading Investments [Member]
Changes in operating assets and liabilities:
Decrease/(increase) in trading investments	0	255
Mutual Funds Held In Rabbi Trust [Member]
Changes in operating assets and liabilities:
Decrease/(increase) in trading investments	$ 837	$ (529)