Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities
Net income	$ 88,427	$ 80,486
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	11,059	9,483
Stock-based compensation expense	7,601	7,468
Deferred taxes	1,448	1,399
Other	1,014	740
Changes in operating assets and liabilities:
(Increase) in accounts receivable	(9,350)	(4,669)
(Increase) in prepaid expenses and other assets	(2,330)	(9,407)
(Decrease) in accrued employee compensation	(12,890)	(11,703)
(Decrease) in income and other tax liabilities	(1,164)	(3,068)
Increase in deferred revenue	935	561
Increase (decrease) in accounts payable, accrued expenses and other liabilities	5,559	(441)
Net cash provided by operating activities	85,818	69,118
Cash flows from investing activities
Proceeds from maturities and sales, available-for-sale investments	152,335	101,354
Purchases, available-for-sale investments	(159,192)	(143,214)
Purchases of furniture, equipment and leasehold improvements	(16,035)	(5,777)
Capitalization of software development costs	(6,536)	(6,667)
Other	16	(33)
Net cash (used in) investing activities	(29,412)	(54,337)
Cash flows from financing activities
Cash dividend on common stock	(31,350)	(24,535)
Exercise of stock options	371	1,338
Withholding tax payments on restricted stock vesting and stock option exercises	(7,997)	(8,111)
Repurchases of common stock	(12,904)	(24,147)
Net cash (used in) financing activities	(51,880)	(55,455)
Effect of exchange rate changes on cash and cash equivalents	(854)	574
Cash and cash equivalents
Net increase (decrease) for the period	3,672	(40,100)
Beginning of period	167,014	168,243
End of period	170,686	128,143
Corporate Debt [Member]
Changes in operating assets and liabilities:
(Increase) in trading securities	(3,288)	(111)
Mutual Funds Held In Rabbi Trust [Member]
Changes in operating assets and liabilities:
(Increase) in trading securities	$ (1,203)	$ (1,620)