Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities
Net income	$ 257,888	$ 299,377	$ 204,902
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	53,447	35,996	26,857
Amortization of operating lease right-of-use assets	6,799	6,842	5,795
Stock-based compensation expense	27,314	25,613	25,294
Deferred taxes	3,118	10,099	2,674
Other	(466)	(550)	(778)
Changes in operating assets and liabilities:
Decrease (Increase) in accounts receivable	15,598	(18,015)	(2,962)
(Increase) in receivables from broker-dealers, clearing organizations and customers	(156,909)	(182,871)	0
Decrease (Increase) in prepaid expenses and other assets	2,214	(1,977)	(4,624)
Increase in accrued employee compensation	(2,607)	14,961	8,312
Increase in payables to broker-dealers, clearing organizations and customers	95,999	133,326	0
(Decrease) Increase in income and other tax liabilities	(5,638)	16,189	187
Increase (decrease) in accounts payable, accrued expenses and other liabilities	215	6,006	(820)
(Decrease) increase in operating lease liabilities	(7,001)	(5,788)	(829)
Net cash provided by operating activities	282,091	404,489	265,935
Cash flows from investing activities
Acquisitions, net of cash and cash equivalents acquired	(17,078)	(23,297)	(97,430)
Proceeds from maturities and sales, available-for-sale investments	0	170,657	170,936
Purchases, available-for-sale investments	0	(32,865)	(160,827)
Purchases of furniture, equipment and leasehold improvements	(17,493)	(15,010)	(12,292)
Capitalization of software development costs	(33,123)	(30,618)	(22,408)
Other	0	0	(30)
Net cash provided by (used in) investing activities	(67,694)	68,867	(122,051)
Cash flows from financing activities
Cash dividend on common stock	(99,792)	(90,566)	(76,231)
Exercise of stock options	7,096	4,007	1,207
Withholding tax payments on restricted stock vesting and stock option exercises	(33,890)	(42,418)	(25,820)
Repurchases of common stock	(63,189)	(16,135)	(17,256)
Proceeds from short-term borrowings	70,348	578,356	0
Repayments of short-term borrowings	(70,348)	(578,356)	0
Net cash (used in) financing activities	(189,775)	(145,112)	(118,100)
Effect of exchange rate changes on cash and cash equivalents	(7,105)	5,553	1,011
Cash and cash equivalents including restricted cash
Net increase for the period	17,517	333,797	26,795
Beginning of period	608,050	274,253	247,458
End of period	625,567	608,050	274,253
Supplemental cash flow information:
Cash paid for income taxes	70,003	45,046	51,766
Cash paid for interest	830	1,142	0
Non-cash investing and financing activity:
Exercise of stock options - cashless	2,750	10,866	1,811
Right-of-use assets obtained in exchange for operating lease liabilities	1,972	727	7,464
Contingent consideration payable recognized in connection with acquisitions	27,947	14,665	0
Liabilities assumed in connection with acquisition:
Fair value of assets acquired	0	0	148,425
Cash paid for acquisition of business, net of cash and cash equivalents acquired	0	0	(97,430)
Treasury stock used for acquisition of business	0	0	(48,830)
Liabilities assumed	0	0	2,165
Trading Investments [Member]
Changes in operating assets and liabilities:
(Increase) decrease in trading investments	(5,574)	67,952	4,045
Mutual Funds Held In Rabbi Trust [Member]
Changes in operating assets and liabilities:
(Increase) decrease in trading investments	$ (2,306)	$ (2,671)	$ (2,118)