Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities
Net income	$ 114,616	$ 93,007
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	14,066	13,546
Stock-based compensation expense	10,989	10,636
Deferred taxes	326	(1,387)
Other	1,290	10,044
Changes in operating assets and liabilities:
(Increase) in accounts receivable	(12,535)	(16,439)
(Increase) in prepaid expenses and other assets	(4,503)	(2,151)
(Decrease) in accrued employee compensation	(3,756)	(750)
(Decrease) in income and other tax liabilities	(3,277)	(843)
Increase in deferred revenue	676	694
(Decrease) increase in accounts payable, accrued expenses and other liabilities	(2,917)	2,646
Net cash provided by operating activities	121,583	33,140
Cash flows from investing activities
Proceeds from maturities and sales, available-for-sale investments	146,295	32,025
Purchases, available-for-sale investments	(196,810)	(42,495)
Purchases of furniture, equipment and leasehold improvements	(7,245)	(4,754)
Capitalization of software development costs	(10,094)	(9,058)
Other	(53)	383
Net cash (used in) investing activities	(67,907)	(23,899)
Cash flows from financing activities
Cash dividend on common stock	(36,717)	(28,914)
Exercise of stock options	1,640	2,172
Withholding tax payments on restricted stock vesting and stock option exercises	(12,906)	(5,929)
Repurchases of common stock	(36,352)	(13,874)
Net cash (used in) financing activities	(84,335)	(46,545)
Effect of exchange rate changes on cash and cash equivalents	1,408	(980)
Cash and cash equivalents
Net (decrease) for the period	(29,251)	(38,284)
Beginning of period	168,243	199,728
End of period	138,992	161,444
Corporate Debt [Member]
Changes in operating assets and liabilities:
Decrease (increase) in trading securities	8,338	(74,535)
Mutual Funds Held In Rabbi Trust [Member]
Changes in operating assets and liabilities:
Decrease (increase) in trading securities	$ (1,730)	$ (1,328)