CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities:
Net income	$ 84,280	$ 63,306
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	16,613	16,591
Deferred income tax (benefit) expense	(3,245)	9,291
Provision for bad debts	64	1,040
Stock-based compensation expense	15,269	10,513
Other	272	(363)
Changes in operating assets and liabilities:
Accounts receivable	22,507	15,060
Other receivables and current assets	6,167	18,203
Other long-term assets	1,189	6,046
Accounts payable, accrued expenses and current liabilities	(40,009)	(23,335)
Accrued income taxes	(1,807)	1,303
Deferred revenue	31,704	7,176
Other long-term liabilities	(583)	1,062
Net cash provided by operating activities	132,421	125,893
Cash flows from investing activities:
Acquisitions, net of cash acquired	0	(5,864)
Capital expenditures	(2,933)	(4,057)
Other investing activities	(4,303)	(964)
Net cash used in investing activities	(7,236)	(10,885)
Cash flows from financing activities:
Purchase of treasury stock	(117,831)	(100,352)
Restricted stock withholding taxes paid in lieu of issued shares	(24,333)	(8,480)
Proceeds from shares issued for stock-based compensation	12,759	30,498
Net cash used in financing activities	(129,405)	(78,334)
Effect of exchange rate fluctuations on cash and cash equivalents	8,212	7,035
Net increase in cash and cash equivalents	3,992	43,709
Cash and cash equivalents, beginning of period	881,501	822,479
Cash and cash equivalents, end of period	885,493	866,188
Supplemental disclosures of cash flow information:
Income taxes paid	$ 3,402	$ 2,376