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	$ 37,825	$ 32,769
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	14,112	12,967
Share-based compensation expense	10,557	8,676
Deferred income tax (benefit)	(2,658)	(3,870)
Changes in operating assets and liabilities, exclusive of effects of acquired companies:
Accounts receivable	30,227	37,542
Income taxes	4,053	7,576
Prepaid expenses and other current assets	1,333	(919)
Accounts payable	(1,752)	(1,166)
Accrued liabilities	(17,952)	(15,939)
Deferred revenue	(31,114)	(29,457)
Net cash provided by operating activities	44,631	48,179
Cash flows from investing activities:
Additions to property and equipment	(8,895)	(19,820)
Purchase of marketable security investments	(43,962)	(7,128)
Proceeds from marketable security investments	11,077	6,896
Decrease (increase) in other	743	(16)
Net cash used by investing activities	(41,037)	(20,068)
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	19,298	14,851
Contributions from employee stock purchase plan	1,798	1,650
Net cash provided (used) by financing activities	21,096	(531)
Net increase in cash and cash equivalents	24,690	27,580
Cash and cash equivalents at beginning of period	185,926	36,151
Cash and cash equivalents at end of period	$ 210,616	$ 63,731