Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities
Net income	$ 226,446	$ 154,629
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	25,404	19,127
Amortization of operating lease right-of-use assets	5,146	4,304
Stock-based compensation expense	18,919	19,069
Deferred taxes	8,655	73
Other	727	886
Changes in operating assets and liabilities:
(Increase) in accounts receivable	(20,232)	(15,479)
(Increase) in receivables from broker-dealers, clearing organizations and customers	(304,023)
Decrease in prepaid expenses and other assets	521	3,855
Decrease (increase) in trading investments	62,636	(371)
Increase (decrease) in accrued employee compensation	3,561	(3,971)
Increase in payables to broker-dealers, clearing organizations and customers	138,120
Increase (decrease) in income and other tax liabilities	10,098	(3,279)
Increase in deferred revenue	1,593	921
Increase (decrease) in accounts payable, accrued expenses and other liabilities	5,786	(2,000)
(Decrease) increase in operating lease liabilities	(4,669)	800
Net cash provided by operating activities	176,869	176,794
Cash flows from investing activities
Acquisition of business, net of cash and cash equivalents acquired	(527)
Proceeds from maturities and sales, available-for-sale investments	170,657	156,809
Purchases, available-for-sale investments	(32,865)	(149,575)
Purchases of furniture, equipment and leasehold improvements	(13,022)	(8,683)
Capitalization of software development costs	(21,124)	(13,863)
Other		22
Net cash provided by (used in) investing activities	103,119	(15,290)
Cash flows from financing activities
Cash dividend on common stock	(68,104)	(57,233)
Exercise of stock options	748	885
Withholding tax payments on restricted stock vesting and stock option exercises	(34,562)	(18,271)
Repurchases of common stock	(15,453)	(11,440)
Proceeds from short-term borrowings	478,356
Repayments of short-term borrowings	(478,356)
Net cash (used in) financing activities	(117,371)	(86,059)
Effect of exchange rate changes on cash and cash equivalents	431	(1,261)
Cash and cash equivalents including restricted cash
Net increase for the period	163,048	74,184
Beginning of period	274,253	247,458
End of period	437,301	321,642
Non-cash activity
Operating lease right-of-use assets obtained in exchange for operating lease liabilities	705	422
Mutual Funds Held In Rabbi Trust [Member]
Changes in operating assets and liabilities:
Decrease (increase) in trading investments	$ (1,819)	$ (1,770)