CONSOLIDATED STATEMENTS OF CASH FLOWS $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Operating activities:
Net income	$ 36,322	$ 33,255	$ 38,043
Adjustment to reconcile net income to cash provided by operating activities:
Excess tax benefits from exercise or vesting of equity awards	(5,275)	(3,387)	(6,435)
Deferred income taxes	(2,099)	(13,896)	(7,008)
Depreciation and amortization	23,093	23,352	20,035
Amortization of investments	2,238	1,916	1,776
Stock-based compensation expense	30,054	19,205	12,869
Foreign currency transaction loss	4,168	3,769	1,593
Other non-cash	822	(747)	1,670
Change in operating assets and liabilities:
Trade accounts receivable	(62,235)	6,209	(28,316)
Income taxes receivable and other current assets	3,223	2,776	7,076
Accounts payable and accrued expenses	16,572	13,356	5,794
Deferred revenue	17,668	11,173	26,362
Other long-term assets and liabilities	(2,023)	2,908	7,244
Cash provided by operating activities	62,528	99,889	80,703
Investing activities:
Purchases of marketable securities	(75,702)	(55,520)	(60,648)
Proceeds from maturities and called marketable securities	42,026	33,244	27,814
Sales of marketable securities	1,915
Payments for acquisitions, net of cash acquired	(1,671)	(3,918)	(25,565)
Investment in property and equipment	(11,020)	(11,463)	(5,598)
Cash used in investing activities	(44,452)	(37,657)	(63,997)
Financing activities:
Issuance of common stock for share-based compensation plans	1,136	1,001	2,210
Excess tax benefits from exercise or vesting of equity awards	5,275	3,387	6,435
Dividend payments to shareholders	(9,194)	(6,874)	(3,421)
Common stock repurchases for tax withholdings for net settlement of equity awards	(9,776)	(6,128)	(7,345)
Common stock repurchases under share repurchase programs	(22,825)	(15,418)	(12,446)
Cash used in financing activities	(35,384)	(24,032)	(14,567)
Effect of exchange rate on cash and cash equivalents	(4,251)	(3,846)	567
Net (decrease) increase in cash and cash equivalents	(21,559)	34,354	2,706
Cash and cash equivalents, beginning of year	114,585	80,231	77,525
Cash and cash equivalents, end of year	93,026	114,585	80,231
Supplemental disclosures:
Income taxes paid	30,215	14,722	11,107
Non-cash investing and financing activity:
Dividends payable	$ 2,297	$ 2,294	$ 1,145