Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net income	$ 148,089	$ 126,172	$ 96,037
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	19,274	17,838	18,542
Stock-based compensation expense	14,421	14,511	12,519
Deferred taxes	6,590	(2,581)	(2,424)
Other	1,953	10,628	6,359
Changes in operating assets and liabilities:
(Increase) in accounts receivable	(2,353)	(10,429)	(6,748)
(Increase) in prepaid expenses and other assets	(7,039)	(2,125)	(2,662)
Increase in accrued employee compensation	1,719	5,487	3,986
Increase (decrease) in income and other tax liabilities	5,770	3,221	(941)
Increase (decrease) in deferred revenue	145	203	(153)
(Decrease) increase in accounts payable, accrued expenses and other liabilities	(1,614)	2,344	(9)
Net cash provided by operating activities	168,035	89,747	124,506
Cash flows from investing activities
Proceeds from maturities and sales, available-for-sale investments	187,794	46,347	35,248
Purchase, available-for-sale investments	(215,831)	(81,438)	(57,175)
Purchases of furniture, equipment and leasehold improvements	(12,086)	(6,385)	(4,795)
Capitalization of software development costs	(13,471)	(12,118)	(10,589)
Other	(59)	414	(642)
Net cash (used in) investing activities	(53,653)	(53,180)	(37,953)
Cash flows from financing activities
Cash dividend on common stock	(48,888)	(38,495)	(29,534)
Exercise of stock options	1,972	2,299	1,824
Withholding tax payments on restricted stock vesting and stock option exercises	(27,691)	(5,173)	(4,496)
Repurchases of common stock	(42,461)	(23,925)	(23,158)
Net cash (used in) financing activities	(117,068)	(65,294)	(55,364)
Effect of exchange rate changes on cash and cash equivalents	1,457	(2,758)	(385)
Cash and cash equivalents
Net (decrease) increase for the period	(1,229)	(31,485)	30,804
Beginning of period	168,243	199,728	168,924
End of period	167,014	168,243	199,728
Supplemental cash flow information:
Cash paid during the year for income taxes	43,997	58,268	$ 50,875
Corporate Debt [Member]
Changes in operating assets and liabilities:
(Increase) in trading securities	(17,081)	(74,195)
Mutual Funds Held In Rabbi Trust [Member]
Changes in operating assets and liabilities:
(Increase) in trading securities	$ (1,839)	$ (1,327)