Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities
Net income	$ 80,457	$ 74,816
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Depreciation and amortization	11,779	8,067
Amortization of operating lease right-of-use assets	1,664	1,645
Stock-based compensation expense	7,424	6,677
Deferred taxes	591	2,734
Other	(74)	2,036
Changes in operating assets and liabilities:
(Increase) in accounts receivable	(7,880)	(31,640)
(Increase) in receivables from broker-dealers, clearing organizations and customers	(270,491)
Decrease in prepaid expenses and other assets	3,082	8,539
Decrease in trading investments	5,495	56,394
(Decrease) in accrued employee compensation	(34,103)	(25,436)
Increase in payables to broker-dealers, clearing organizations and customers	174,942
Increase in income and other tax liabilities	4,814	1,950
Increase in accounts payable, accrued expenses and other liabilities	2,599	16,392
(Decrease) in operating lease liabilities	(1,845)	(1,516)
Net cash (used in) provided by operating activities	(23,159)	120,369
Cash flows from investing activities
Acquisition of business		(533)
Proceeds from maturities and sales, available-for-sale investments		45,154
Purchases, available-for-sale investments		(8,995)
Purchases of furniture, equipment and leasehold improvements	(4,257)	(4,291)
Capitalization of software development costs	(8,075)	(6,778)
Net cash (used in) provided by investing activities	(12,332)	24,557
Cash flows from financing activities
Cash dividend on common stock	(25,454)	(23,244)
Exercise of stock options	244	80
Withholding tax payments on restricted stock vesting and stock option exercises	(27,422)	(21,243)
Repurchases of common stock	(520)	(5,415)
Proceeds from short-term borrowings	69,302
Repayments of short-term borrowings	(35,000)
Net cash (used in) financing activities	(18,850)	(49,822)
Effect of exchange rate changes on cash and cash equivalents	(1,027)	(3,360)
Cash and cash equivalents including restricted cash
Net (decrease) increase for the period	(55,368)	91,744
Beginning of period	608,050	274,253
End of period	552,682	365,997
Non-cash activity
Operating lease right-of-use assets obtained in exchange for operating lease liabilities	878	674
Mutual Funds Held In Rabbi Trust [Member]
Changes in operating assets and liabilities:
Decrease in trading investments	$ (1,613)	$ (289)