CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities:
Net income	$ 63,884	$ 43,231
Adjustments to reconcile net income to cash provided by operations:
Depreciation and amortization	26,366	24,850
Share-based compensation expense	17,577	13,692
Deferred income tax (benefit) expense	(9,097)	427
Changes in operating assets and liabilities, exclusive of effects of acquired companies:
Accounts receivable	(33,348)	(35,530)
Income taxes	(9,311)	(3,500)
Prepaid expenses and other current assets	(3,984)	(1,435)
Accounts payable	2,152	(236)
Accrued liabilities	(11,061)	4,883
Deferred revenue	6,397	14,459
Net cash provided by operating activities	49,575	60,841
Cash flows from investing activities:
Additions to property and equipment	(30,123)	(21,959)
Purchase of marketable security investments	(21,392)	(10,607)
Proceeds from marketable security investments	17,029	6,526
Cost of acquisitions, net of cash acquired	(5,855)	(9,394)
Increase in other	(68)	(281)
Net cash used by investing activities	(40,409)	(35,715)
Cash flows from financing activities:
(Decrease) increase in net borrowings on revolving line of credit	(10,000)	69,000
Purchase of treasury shares	(7,032)	(94,497)
Proceeds from exercise of stock options	23,360	5,793
Contributions from employee stock purchase plan	3,427	2,818
Net cash provided (used) by financing activities	9,755	(16,886)
Net increase in cash and cash equivalents	18,921	8,240
Cash and cash equivalents at beginning of period	36,151	33,087
Cash and cash equivalents at end of period	$ 55,072	$ 41,327