Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net income	$ 204,902	$ 172,852	$ 148,089
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	26,857	23,080	19,274
Amortization of operating lease right-of-use assets	5,795
Stock-based compensation expense	25,294	15,850	14,421
Deferred taxes	2,682	(1,258)	6,590
Other	(786)	2,338	1,953
Changes in operating assets and liabilities:
(Increase) in accounts receivable	(2,962)	(5,117)	(2,353)
(Increase) in prepaid expenses and other assets	(4,624)	(3,791)	(7,039)
Decrease (Increase) in trading securities	4,045	856	(17,081)
Increase in accrued employee compensation	8,312	2,551	1,719
Increase in income and other tax liabilities	187	2,965	5,770
Increase in deferred revenue	689	150	145
(Decrease) increase in accounts payable, accrued expenses and other liabilities	(1,509)	14,374	(1,614)
(Decrease) in operating lease liabilities	(829)
Net cash provided by operating activities	265,935	223,917	168,035
Cash flows from investing activities
Acquisition of business, net of cash and cash equivalents acquired	(97,430)
Proceeds from maturities and sales, available-for-sale investments	170,936	333,791	187,794
Purchases, available-for-sale investments	(160,827)	(336,533)	(215,831)
Purchases of furniture, equipment and leasehold improvements	(12,292)	(35,888)	(12,086)
Capitalization of software development costs	(22,408)	(11,705)	(13,471)
Other	(30)	39	(59)
Net cash (used in) investing activities	(122,051)	(50,296)	(53,653)
Cash flows from financing activities
Cash dividend on common stock	(76,231)	(62,432)	(48,888)
Exercise of stock options	1,207	2,973	1,972
Withholding tax payments on restricted stock vesting and stock option exercises	(25,820)	(8,043)	(27,691)
Repurchases of common stock	(17,256)	(25,171)	(42,461)
Net cash (used in) financing activities	(118,100)	(92,673)	(117,068)
Effect of exchange rate changes on cash and cash equivalents	1,011	(1,640)	1,457
Cash and cash equivalents including restricted cash
Net increase (decrease) for the period	26,795	79,308	(1,229)
Beginning of period	247,458	168,150	169,379
End of period	274,253	247,458	168,150
Supplemental cash flow information:
Cash paid during the year for income taxes	51,766	47,208	43,997
Right-of-use assets obtained in exchange for operating lease liabilities	7,464
Non-cash investing and financing activity:
Fair value of assets acquired	148,425
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	$ (2,118)	$ (933)	$ (1,839)