Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net income	$ 172,852	$ 148,089	$ 126,172
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	23,080	19,274	17,838
Stock-based compensation expense	15,850	14,421	14,511
Deferred taxes	(1,258)	6,590	(2,581)
Other	2,338	1,953	10,628
Changes in operating assets and liabilities:
(Increase) in accounts receivable	(5,117)	(2,353)	(10,429)
(Increase) in prepaid expenses and other assets	(3,791)	(7,039)	(2,125)
Decrease (Increase) in trading securities	856	(17,081)	(74,195)
Increase in accrued employee compensation	2,551	1,719	5,487
Increase in income and other tax liabilities	2,965	5,770	3,221
Increase in deferred revenue	150	145	203
Increase (decrease) in accounts payable, accrued expenses and other liabilities	14,374	(1,614)	2,344
Net cash provided by operating activities	223,917	168,035	89,747
Cash flows from investing activities
Proceeds from maturities and sales, available-for-sale investments	333,791	187,794	46,347
Purchases, available-for-sale investments	(336,533)	(215,831)	(81,438)
Purchases of furniture, equipment and leasehold improvements	(35,888)	(12,086)	(6,385)
Capitalization of software development costs	(11,705)	(13,471)	(12,118)
Other	39	(59)	414
Net cash (used in) investing activities	(50,296)	(53,653)	(53,180)
Cash flows from financing activities
Cash dividend on common stock	(62,432)	(48,888)	(38,495)
Exercise of stock options	2,973	1,972	2,299
Withholding tax payments on restricted stock vesting and stock option exercises	(8,043)	(27,691)	(5,173)
Repurchases of common stock	(25,171)	(42,461)	(23,925)
Net cash (used in) financing activities	(92,673)	(117,068)	(65,294)
Effect of exchange rate changes on cash and cash equivalents	(1,640)	1,457	(2,758)
Cash and cash equivalents including restricted cash
Net increase (decrease) for the period	79,308	(1,229)	(31,485)
Beginning of period	168,150	169,379	200,864
End of period	247,458	168,150	169,379
Supplemental cash flow information:
Cash paid during the year for income taxes	47,208	43,997	58,268
Mutual Funds Held In Rabbi Trust [Member]
Changes in operating assets and liabilities:
Decrease (Increase) in trading securities	$ (933)	$ (1,839)	$ (1,327)