CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities:
Net income	$ 102,147	$ 78,661
Adjustments to reconcile net income to cash provided by operations:
Depreciation and amortization	40,096	37,521
Share-based compensation expense	27,368	21,348
Deferred income tax benefit	(14,216)	(11,289)
Changes in operating assets and liabilities, exclusive of effects of acquired companies:
Accounts receivable	(7,097)	(14,641)
Income taxes	(467)	1,769
Prepaid expenses and other current assets	(2,706)	1,169
Accounts payable	(2,733)	(917)
Accrued liabilities	1,318	8,515
Deferred revenue	(1,329)	17,918
Net cash provided by operating activities	142,381	140,054
Cash flows from investing activities:
Additions to property and equipment	(37,734)	(29,529)
Purchase of marketable security investments	(49,905)	(13,127)
Proceeds from marketable security investments	21,175	9,256
Cost of acquisitions, net of cash acquired	(9,761)	(9,394)
Decrease (increase) in other	418	(52)
Net cash used by investing activities	(75,807)	(42,846)
Cash flows from financing activities:
Decrease in net borrowings on revolving line of credit	(10,000)	(32,000)
Purchase of treasury shares	(7,032)	(94,499)
Proceeds from exercise of stock options	33,568	15,089
Contributions from employee stock purchase plan	5,342	4,429
Net cash provided (used) by financing activities	21,878	(106,981)
Net increase (decrease) in cash and cash equivalents	88,452	(9,773)
Cash and cash equivalents at beginning of period	36,151	33,087
Cash and cash equivalents at end of period	$ 124,603	$ 23,314