CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net income	$ 265,636	$ 252,521	$ 254,690
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	69,587	77,670	81,924
Deferred income tax benefit	(10,921)	(15,196)	(18,859)
Provision for bad debts	2,009	1,304	2,104
Stock-based compensation expense	33,347	33,951	36,861
Excess tax benefits from stock-based compensation	(9,153)	(8,176)	(14,531)
Other	1,290	1,413	868
Changes in operating assets and liabilities:
Accounts receivable	(17,388)	6,044	(5,554)
Other receivables and current assets	(39,644)	(17,662)	(877)
Other long-term assets	(7,167)	273	(1,838)
Accounts payable, accrued expenses and current liabilities	16,919	(6,993)	8,208
Accrued income taxes	9,052	5,770	12,102
Deferred revenue	41,430	40,566	35,548
Other long-term liabilities	1,830	(3,962)	(5,339)
Net cash provided by operating activities	356,827	367,523	385,307
Cash flows from investing activities:
Acquisitions, net of cash acquired	(7,891)	(46,117)	(103,016)
Capital expenditures	(12,443)	(16,145)	(26,023)
Other investing activities	(11,839)	230	(231)
Net cash used in investing activities	(32,173)	(62,032)	(129,270)
Cash flows from financing activities:
Purchase of treasury stock	(336,335)	(337,910)	(233,793)
Restricted stock withholding taxes paid in lieu of issued shares	(5,057)	(4,446)	(5,108)
Contingent consideration payments	(1,048)	(1,173)	(4,504)
Proceeds from shares issued for stock-based compensation	53,811	43,623	43,323
Excess tax benefits from stock-based compensation	9,153	8,176	14,531
Other financing activities	(1)	(21)	(91)
Net cash used in financing activities	(279,477)	(291,751)	(185,642)
Effect of exchange rate fluctuations on cash and cash equivalents	(6,866)	(17,636)	(24,817)
Net increase (decrease) in cash and cash equivalents	38,311	(3,896)	45,578
Cash and cash equivalents, beginning of period	784,168	788,064	742,486
Cash and cash equivalents, end of period	822,479	784,168	788,064
Supplemental disclosures of cash flow information:
Income taxes paid	118,455	107,218	118,004
Interest paid	822	620	643
Fair value of stock options and restricted stock awards assumed in connection with acquisitions	0	3,528	68
Construction-in-progress - leased facility	$ 0	$ 0	$ (18,136)