Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Operating activities:
Net income	$ 9,765	$ 9,069
Adjustments to reconcile net income to cash provided by operating activities:
Excess tax benefits from exercise or vesting of equity awards	(971)	(725)
Deferred income taxes	44	56
Depreciation and amortization	5,846	4,727
Stock-based compensation expense	3,295	3,432
Foreign currency transaction (gain) loss	(322)	1,890
Other non-cash items	222	1,241
Change in operating assets and liabilities:
Trade accounts receivable	57,291	61,743
Income taxes receivable and other current assets	1,629	1,569
Accounts payable and accrued expenses	(21,587)	(23,978)
Deferred revenue	18,337	6,811
Other long-term assets and liabilities	(691)	211
Cash provided by operating activities	72,858	66,046
Investing activities:
Purchase of marketable securities	(11,630)	(15,779)
Matured and called marketable securities	11,021	3,750
Payments for prior year acquisition	(793)
Investment in property and equipment	(1,228)	(1,195)
Cash used in investing activities	(2,630)	(13,224)
Financing activities:
Issuance of common stock for share-based compensation plans	22	271
Excess tax benefits from exercise or vesting of equity awards	971	725
Dividend payments to shareholders	(1,145)
Common stock repurchases for tax withholdings for net settlement of equity awards	(1,805)	(1,611)
Common stock repurchases under share repurchase programs	(4,630)	(3,512)
Cash used in financing activities	(6,587)	(4,127)
Effect of exchange rate on cash and cash equivalents	458	(2,489)
Net increase in cash and cash equivalents	64,099	46,206
Cash and cash equivalents, beginning of period	80,231	77,525
Cash and cash equivalents, end of period	$ 144,330	$ 123,731