Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Cash flows from operating activities
Net income	$ 147,745	$ 158,670
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	24,876	16,372
Amortization of operating lease right-of-use assets	3,368	3,325
Stock-based compensation expense	13,558	12,468
Deferred taxes	1,600	4,741
Other	(176)	2,199
Changes in operating assets and liabilities:
(Increase) in accounts receivable	(7,434)	(37,457)
(Increase) in receivables from broker-dealers, clearing organizations and customers	(211,434)
(Increase) decrease in prepaid expenses and other assets	(4,624)	6,124
(Increase) decrease in trading investments	(5,569)	63,274
(Decrease) in accrued employee compensation	(23,048)	(7,931)
Increase in payables to broker-dealers, clearing organizations and customers	144,977
(Decrease) increase in income and other tax liabilities	(3,101)	3,839
(Decrease) increase in accounts payable, accrued expenses and other liabilities	(1,479)	4,029
(Decrease) in operating lease liabilities	(3,732)	(3,072)
Net cash provided by operating activities	73,567	225,222
Cash flows from investing activities
Acquisitions, net of cash and cash equivalents acquired	(17,078)	(533)
Proceeds from maturities and sales, available-for-sale investments		68,596
Purchases, available-for-sale investments		(32,865)
Purchases of furniture, equipment and leasehold improvements	(9,809)	(9,264)
Capitalization of software development costs	(16,459)	(13,003)
Net cash (used in) provided by investing activities	(43,346)	12,931
Cash flows from financing activities
Cash dividend on common stock	(50,238)	(45,668)
Exercise of stock options	4,665	739
Withholding tax payments on restricted stock vesting and stock option exercises	(34,282)	(27,778)
Repurchases of common stock	(13,956)	(11,288)
Proceeds from short-term borrowings	70,348
Repayments of short-term borrowings	(69,302)
Net cash (used in) financing activities	(92,765)	(83,995)
Effect of exchange rate changes on cash and cash equivalents	(263)	(3,678)
Cash and cash equivalents including restricted cash
Net (decrease) increase for the period	(62,807)	150,480
Beginning of period	608,050	274,253
End of period	545,243	424,733
Supplemental cash flow information
Cash paid for income taxes	36,798	24,068
Cash paid for interest	362
Non-cash activity
Contingent consideration payable assumed in connection with acquisition	22,450
Exercise of stock options - cashless	2,750	5,119
Operating lease right-of-use assets obtained in exchange for operating lease liabilities	1,208	688
Mutual Funds Held In Rabbi Trust [Member]
Changes in operating assets and liabilities:
(Increase) decrease in trading investments	$ (1,960)	$ (1,359)