Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flows from operating activities
Net income	$ 158,670	$ 100,627
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	16,372	12,427
Amortization of operating lease right-of-use assets	3,325	2,835
Stock-based compensation expense	12,468	11,125
Deferred taxes	4,741	1,631
Other	2,199	(296)
Changes in operating assets and liabilities:
(Increase) in accounts receivable	(37,457)	(8,883)
Decrease (increase) in prepaid expenses and other assets	6,124	(131)
Decrease in trading investments	63,274	8,854
(Decrease) in accrued employee compensation	(7,931)	(12,384)
Increase (decrease) in income and other tax liabilities	3,839	(3,661)
Increase in deferred revenue	569	1,000
Increase (decrease) in accounts payable, accrued expenses and other liabilities	3,460	(877)
(Decrease) in operating lease liabilities	(3,072)	(1,422)
Net cash provided by operating activities	225,222	109,102
Cash flows from investing activities
Acquisition of business, net of cash and cash equivalents acquired	(533)
Proceeds from maturities and sales, available-for-sale investments	68,596	130,875
Purchases, available-for-sale investments	(32,865)	(128,189)
Purchases of furniture, equipment and leasehold improvements	(9,264)	(6,114)
Capitalization of software development costs	(13,003)	(7,310)
Other		2
Net cash provided by (used in) investing activities	12,931	(10,736)
Cash flows from financing activities
Cash dividend on common stock	(45,668)	(38,323)
Exercise of stock options	739	522
Withholding tax payments on restricted stock vesting and stock option exercises	(27,778)	(12,061)
Repurchases of common stock	(11,288)	(8,670)
Net cash (used in) financing activities	(83,995)	(58,532)
Effect of exchange rate changes on cash and cash equivalents	(3,678)	21
Cash and cash equivalents including restricted cash
Net increase for the period	150,480	39,855
Beginning of period	274,253	247,458
End of period	424,733	287,313
Non-cash activity
Operating lease right-of-use assets obtained in exchange for operating lease liabilities	688
Mutual Funds Held In Rabbi Trust [Member]
Changes in operating assets and liabilities:
Decrease in trading investments	$ (1,359)	$ (1,743)