CONSOLIDATED STATEMENTS OF CASH FLOWS $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Operating activities:
Net income	$ 26,986	$ 36,322	$ 33,255
Adjustment to reconcile net income to cash provided by operating activities:
Deferred income taxes	(5,810)	(2,099)	(13,896)
Depreciation and amortization	24,137	23,093	23,352
Amortization of investments	1,862	2,238	1,916
Stock-based compensation expense	40,821	30,054	19,205
Foreign currency transaction (gain) loss	(2,247)	4,168	3,769
Other non-cash	(1,382)	822	(747)
Change in operating assets and liabilities:
Trade accounts receivable	(56,730)	(62,235)	6,209
Income taxes receivable and other current assets	(10,818)	3,223	2,776
Accounts payable and accrued expenses	1,531	16,572	13,356
Deferred revenue	21,271	17,668	11,173
Other long-term assets and liabilities	253	(2,023)	2,908
Cash provided by operating activities	39,874	67,803	103,276
Investing activities:
Purchases of marketable securities	(23,969)	(75,702)	(55,520)
Proceeds from maturities and called marketable securities	22,788	42,026	33,244
Sales of marketable securities	62,210	1,915
Payments for acquisitions, net of cash acquired	(49,113)	(1,671)	(3,918)
Investment in property and equipment	(19,088)	(11,020)	(11,463)
Cash used in investing activities	(7,172)	(44,452)	(37,657)
Financing activities:
Issuance of common stock for share-based compensation plans	889	1,136	1,001
Dividend payments to shareholders	(9,174)	(9,194)	(6,874)
Common stock repurchases for tax withholdings for net settlement of equity awards	(16,183)	(9,776)	(6,128)
Common stock repurchases under share repurchase programs	(27,248)	(22,825)	(15,418)
Cash used in financing activities	(51,716)	(40,659)	(27,419)
Effect of exchange rate on cash and cash equivalents	(3,418)	(4,251)	(3,846)
Net (decrease) increase in cash and cash equivalents	(22,432)	(21,559)	34,354
Cash and cash equivalents, beginning of year	93,026	114,585	80,231
Cash and cash equivalents, end of year	70,594	93,026	114,585
Supplemental disclosures:
Income taxes paid	28,844	30,215	14,722
Non-cash investing and financing activity:
Dividends payable	$ 2,298	$ 2,297	$ 2,294