Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities
Net income	$ 47,940	$ 42,463
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	5,269	4,693
Stock-based compensation expense	3,951	3,943
Deferred taxes	2,061	1,827
Other	(186)	393
Changes in operating assets and liabilities:
(Increase) in accounts receivable	(14,955)	(6,113)
Decrease (increase) in prepaid expenses and other assets	852	(2,146)
(Decrease) in accrued employee compensation	(20,891)	(20,911)
Increase in income and other tax liabilities	4,304	7,366
Increase in deferred revenue	894	675
Increase in accounts payable, accrued expenses and other liabilities	307	1,074
Net cash provided by operating activities	31,439	30,549
Cash flows from investing activities
Proceeds from maturities and sales, available-for-sale investments	51,002	46,485
Purchases, available-for-sale investments	(57,839)	(68,166)
Purchases of furniture, equipment and leasehold improvements	(2,278)	(4,149)
Capitalization of software development costs	(3,590)	(3,172)
Other	(24)	(5)
Net cash (used in) investing activities	(12,729)	(29,007)
Cash flows from financing activities
Cash dividend on common stock	(15,814)	(12,356)
Exercise of stock options	258	500
Withholding tax payments on restricted stock vesting and stock option exercises	(7,621)	(7,642)
Repurchases of common stock	(6,268)	(12,207)
Net cash (used in) financing activities	(29,445)	(31,705)
Effect of exchange rate changes on cash and cash equivalents	(110)	(425)
Cash and cash equivalents
Net (decrease) for the period	(10,845)	(30,588)
Beginning of period	167,014	168,243
End of period	156,169	137,655
Corporate Debt [Member]
Changes in operating assets and liabilities:
Decrease (increase) in trading securities	3,054	(1,196)
Mutual Funds Held In Rabbi Trust [Member]
Changes in operating assets and liabilities:
Decrease (increase) in trading securities	$ (1,161)	$ (1,519)