Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Cash flows from operating activities
Net income	$ 250,224	$ 257,888	$ 299,377
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	61,446	53,447	35,996
Amortization of operating lease right-of-use assets	5,708	6,799	6,842
Stock-based compensation expense	29,864	27,314	25,613
Deferred taxes	(6,547)	3,118	10,099
Foreign currency transaction gains	(8,783)	0	0
Other	555	(466)	(550)
Changes in operating assets and liabilities:
(Increase) decrease in accounts receivable	(15,136)	15,598	(18,015)
(Increase) in receivables from broker-dealers, clearing organizations and customers	(47,631)	(156,909)	(182,871)
(Increase) in prepaid expenses and other assets	(4,249)	2,214	(1,977)
(Decrease) increase in accrued employee compensation	(3,417)	(2,607)	14,961
Increase in payables to broker-dealers, clearing organizations and customers	74,668	95,999	133,326
(Decrease) Increase in income and other tax liabilities	(4,768)	(5,638)	16,189
Increase in accounts payable, accrued expenses and other liabilities	11,384	215	6,006
(Decrease) in operating lease liabilities	(6,373)	(7,001)	(5,788)
Net cash provided by operating activities	289,231	282,091	404,489
Cash flows from investing activities
Acquisitions, net of cash and cash equivalents acquired	0	(17,078)	(23,297)
Acquisition of equity method investment	(34,400)	0	0
Proceeds from maturities and sales, available-for-sale investments	0	0	170,657
Purchases	0	0	(32,865)
Purchases of furniture, equipment and leasehold improvements	(13,142)	(17,493)	(15,010)
Capitalization of software development costs	(38,730)	(33,123)	(30,618)
Net cash (used in) provided by investing activities	(86,272)	(67,694)	68,867
Cash flows from financing activities
Cash dividend on common stock	(105,942)	(99,792)	(90,566)
Exercise of stock options	672	7,096	4,007
Withholding tax payments on restricted stock vesting and stock option exercises	(23,404)	(33,890)	(42,418)
Repurchases of common stock	(87,540)	(63,189)	(16,135)
Payment of contingent consideration	(26,164)	0	0
Proceeds from short-term borrowings	100,000	70,348	578,356
Repayments of short-term borrowings	(100,000)	(70,348)	(578,356)
Net cash (used in) financing activities	(242,378)	(189,775)	(145,112)
Effect of exchange rate changes on cash and cash equivalents	(13,484)	(7,105)	5,553
Cash and cash equivalents including restricted cash
Net increase (decrease) for the period	(52,903)	17,517	333,797
Beginning of period	625,567	608,050	274,253
End of period	572,664	625,567	608,050
Supplemental cash flow information:
Cash paid for income taxes	88,677	70,003	45,046
Cash paid for interest	652	830	1,142
Non-cash investing and financing activity:
Exercise of stock options - cashless	3,845	2,750	10,866
Right-of-use assets obtained in exchange for operating lease liabilities	1,880	1,972	727
Contingent consideration payable recognized in connection with acquisitions	0	27,947	14,665
Trading Investments [Member]
Changes in operating assets and liabilities:
(Increase) decrease in trading investments	(49,527)	(5,574)	67,952
Mutual Funds Held In Rabbi Trust [Member]
Changes in operating assets and liabilities:
(Increase) decrease in trading investments	$ 1,813	$ (2,306)	$ (2,671)