CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities:
Net income	$ 115,910	$ 103,375
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	45,627	39,700
Share-based compensation expense	37,966	27,368
Deferred income tax benefit	(5,034)	(13,705)
Changes in operating assets and liabilities, exclusive of effects of acquired companies:
Accounts receivable	(31,393)	(6,672)
Income taxes	(1,942)	(467)
Prepaid expenses and other current assets	983	(3,663)
Accounts payable	(4,729)	(2,733)
Accrued liabilities	1,523	1,316
Deferred revenue	20,442	(2,138)
Net cash provided by operating activities	179,353	142,381
Cash flows from investing activities:
Additions to property and equipment	(23,460)	(37,734)
Purchase of marketable security investments	(92,638)	(49,905)
Proceeds from marketable security investments	60,208	21,175
Cost of acquisitions, net of cash acquired	(167,308)	(9,761)
Decrease in other	857	418
Net cash used by investing activities	(222,341)	(75,807)
Cash flows from financing activities:
Decrease in net borrowings on revolving line of credit	0	(10,000)
Purchase of treasury shares	0	(7,032)
Proceeds from exercise of stock options	70,536	33,568
Contributions from employee stock purchase plan	5,978	5,342
Net cash provided by financing activities	76,514	21,878
Net increase in cash and cash equivalents	33,526	88,452
Cash and cash equivalents at beginning of period	185,926	36,151
Cash and cash equivalents at end of period	$ 219,452	$ 124,603