CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 259,251	$ 265,636	$ 252,521
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	67,678	69,587	77,670
Deferred income tax benefit	(2,693)	(10,921)	(15,196)
Provision for bad debts	1,474	2,009	1,304
Stock-based compensation expense	53,154	33,347	33,951
Other	21	1,290	1,413
Changes in operating assets and liabilities:
Accounts receivable	(14,406)	(17,388)	6,044
Other receivables and current assets	(18,498)	(39,644)	(17,662)
Other long-term assets	2,343	(7,167)	273
Accounts payable, accrued expenses and current liabilities	27,045	16,919	(6,993)
Accrued income taxes	1,215	9,052	5,770
Deferred revenue	20,648	41,430	40,566
Other long-term liabilities	33,206	1,830	(3,962)
Net cash provided by operating activities	430,438	365,980	375,699
Cash flows from investing activities:
Acquisitions, net of cash acquired	(63,885)	(7,891)	(46,117)
Capital expenditures	(19,149)	(12,443)	(16,145)
Other investing activities	(14,409)	(11,839)	230
Net cash used in investing activities	(97,443)	(32,173)	(62,032)
Cash flows from financing activities:
Purchase of treasury stock	(336,042)	(336,335)	(337,910)
Restricted stock withholding taxes paid in lieu of issued shares	(11,112)	(5,057)	(4,446)
Contingent consideration payments	0	(1,048)	(1,173)
Proceeds from shares issued for stock-based compensation	52,503	53,811	43,623
Other financing activities	0	(1)	(21)
Net cash used in financing activities	(294,651)	(288,630)	(299,927)
Effect of exchange rate fluctuations on cash and cash equivalents	20,678	(6,866)	(17,636)
Net increase (decrease) in cash and cash equivalents	59,022	38,311	(3,896)
Cash and cash equivalents, beginning of period	822,479	784,168	788,064
Cash and cash equivalents, end of period	881,501	822,479	784,168
Supplemental disclosures of cash flow information:
Income taxes paid	116,389	118,455	107,218
Interest paid	199	822	620
Fair value of stock options and restricted stock awards assumed in connection with acquisitions	$ 0	$ 0	$ 3,528