Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Cash flows from operating activities
Net Income (Loss)	$ 72,615	$ 73,628
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	18,200	16,461
Amortization of operating lease right-of-use assets	1,599	1,368
Stock-based compensation expense	7,103	7,488
Deferred taxes	(1,338)	(1,433)
Foreign currency transaction losses	187	1,699
Other	1,730	(2,532)
Changes in operating assets and liabilities:
(Increase) in accounts receivable	(10,491)	(21,935)
Decrease/(increase) in receivables from broker-dealers, clearing organizations and customers	21,735	(52,549)
(Increase)/decrease in prepaid expenses and other assets	(1,323)	3,163
(Decrease) in accrued employee compensation	(29,017)	(31,567)
(Decrease)/increase in payables to broker-dealers, clearing organizations and customers	(71,135)	12,281
(Decrease)/increase in income and other tax liabilities	(4,001)	10,904
(Decrease) in accounts payable, accrued expenses and other liabilities	(8,385)	(6,838)
(Decrease) in operating lease liabilities	(2,154)	(1,562)
Net cash (used in)/ provided by operating activities	(4,949)	7,527
Cash flows from investing activities
Proceeds from maturities and sales	4,302	1,048
Purchases	(4,972)	(22,895)
Purchases of furniture, equipment and leasehold improvements	(1,197)	(217)
Capitalization of software development costs	(13,963)	(10,690)
Net cash (used in) investing activities	(15,830)	(32,754)
Cash flows from financing activities
Cash dividend on common stock	(29,480)	(28,357)
Exercise of stock options	1,977	707
Withholding tax payments on full value awards vesting and stock option exercises	(14,893)	(20,492)
Repurchases of common stock	(10,147)	0
Proceeds from short-term borrowings	0	50,000
Repayments of short-term borrowings	0	(50,000)
Net cash (used in) financing activities	(52,543)	(48,142)
Effect of exchange rate changes on cash and cash equivalents	(3,186)	3,345
Cash and cash equivalents including restricted cash
Net decrease for the period	(76,508)	(70,024)
Beginning of period	611,672	572,664
End of period	535,164	502,640
Supplemental cash flow information
Cash paid for income taxes	37,187	11,660
Cash paid for interest	431	187
Non-cash investing and financing activity
Operating lease right-of-use assets obtained in exchange for operating lease liabilities	630	0
Furniture, equipment, software and leasehold improvement additions included in accounts payable	4,825	0
Stock-based and accrued incentive compensation relating to capitalized software development costs	921	0
Exercise of stock options - cashless	1,735	0
Trading Investments [Member]
Changes in operating assets and liabilities:
Decrease/(increase) in trading investments	255	(419)
Mutual Funds Held In Rabbi Trust [Member]
Changes in operating assets and liabilities:
Decrease/(increase) in trading investments	$ (529)	$ (630)