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