Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities
Net income	$ 205,703	$ 226,446
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	38,840	25,404
Amortization of operating lease right-of-use assets	5,055	5,146
Stock-based compensation expense	20,312	18,919
Deferred taxes	3,038	8,655
Other	(46)	727
Changes in operating assets and liabilities:
Decrease (Increase) in accounts receivable	11,820	(20,232)
(Increase) in receivables from broker-dealers, clearing organizations and customers	(288,107)	(304,023)
(Increase) decrease in prepaid expenses and other assets	(1,915)	521
(Increase) decrease in trading investments	(5,569)	62,636
(Decrease) Increase in accrued employee compensation	(13,280)	3,561
Increase in payables to broker-dealers, clearing organizations and customers	170,113	138,120
(Decrease) increase in income and other tax liabilities	(914)	10,098
(Decrease) increase in accounts payable, accrued expenses and other liabilities	(1,418)	7,379
(Decrease) in operating lease liabilities	(5,338)	(4,669)
Net cash provided by operating activities	136,380	176,869
Cash flows from investing activities
Acquisitions, net of cash and cash equivalents acquired	(17,078)	(527)
Proceeds from maturities and sales, available-for-sale investments	0	170,657
Purchases, available-for-sale investments	0	(32,865)
Purchases of furniture, equipment and leasehold improvements	(14,567)	(13,022)
Capitalization of software development costs	(24,650)	(21,124)
Net cash (used in) provided by investing activities	(56,295)	103,119
Cash flows from financing activities
Cash dividend on common stock	(74,999)	(68,104)
Exercise of stock options	6,948	748
Withholding tax payments on restricted stock vesting and stock option exercises	(33,371)	(34,562)
Repurchases of common stock	(18,192)	(15,453)
Proceeds from short-term borrowings	70,348	478,356
Repayments of short-term borrowings	(70,348)	(478,356)
Net cash (used in) financing activities	(119,614)	(117,371)
Effect of exchange rate changes on cash and cash equivalents	(4,720)	431
Cash and cash equivalents including restricted cash
Net (decrease) increase for the period	(44,249)	163,048
Beginning of period	608,050	274,253
End of period	563,801	437,301
Supplemental cash flow information
Cash paid for income taxes	48,669	38,488
Cash paid for interest	676	1,046
Non-cash activity
Contingent consideration payable assumed in connection with acquisition	26,898	0
Exercise of stock options - cashless	2,750	10,288
Operating lease right-of-use assets obtained in exchange for operating lease liabilities	1,891	705
Mutual Funds Held In Rabbi Trust [Member]
Changes in operating assets and liabilities:
(Increase) decrease in trading investments	$ (1,914)	$ (1,819)