CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net income	$ 252,521	$ 254,690	$ 245,327
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	77,670	81,924	80,701
Deferred income tax benefit	(15,196)	(18,859)	(24,025)
Provision for bad debts	1,304	2,104	1,465
Stock-based compensation expense	33,951	36,861	35,298
Excess tax benefits from stock-based compensation	(8,176)	(14,531)	(9,971)
Other	1,413	868	73
Changes in operating assets and liabilities:
Accounts receivable	6,044	(5,554)	(2,983)
Other receivables and current assets	(17,662)	(877)	(44,162)
Other long-term assets	273	(1,838)	(462)
Accounts payable, accrued expenses and current liabilities	(6,993)	8,208	15,737
Accrued income taxes	5,770	12,102	11,876
Deferred revenue	40,566	35,548	42,105
Other long-term liabilities	(3,962)	(5,339)	(17,996)
Net cash provided by operating activities	367,523	385,307	332,983
Cash flows from investing activities:
Acquisitions, net of cash acquired	(46,117)	(103,016)	(4,224)
Capital expenditures	(16,145)	(26,023)	(28,848)
Other investing activities	230	(231)	(105)
Net cash used in investing activities	(62,032)	(129,270)	(33,177)
Cash flows from financing activities:
Principal payments on long-term debt and capital leases	(21)	(91)	(53,149)
Purchase of treasury stock	(337,910)	(233,793)	(116,132)
Restricted stock withholding taxes paid in lieu of issued shares	(4,446)	(5,108)	(4,269)
Contingent consideration payments	(1,173)	(4,504)	(3,174)
Proceeds from shares issued for stock-based compensation	43,623	43,323	36,994
Excess tax benefits from stock-based compensation	8,176	14,531	9,971
Net cash used in financing activities	(291,751)	(185,642)	(129,759)
Effect of exchange rate fluctuations on cash and cash equivalents	(17,636)	(24,817)	(4,264)
Net (decrease) increase in cash and cash equivalents	(3,896)	45,578	165,783
Cash and cash equivalents, beginning of period	788,064	742,486	576,703
Cash and cash equivalents, end of period	784,168	788,064	742,486
Supplemental disclosures of cash flow information:
Income taxes paid	107,218	118,004	97,706
Interest paid	620	643	736
Fair value of stock options and restricted stock awards assumed in connection with acquisitions	3,528	68	0
Construction-in-progress - leased facility	$ 0	$ (18,136)	$ 18,136