Note 2 - Significant Accounting Policies (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Restricted Cash and Cash Equivalents, Noncurrent	$ 488	$ 468
Allocated Share-based Compensation Expense	$ 12,938	7,794	$ 4,664
Minimum Tax Percentage Attained Likely to be Realized upon Ultimate Settlement	50.00%
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent	8.33%
Severance Costs	$ 3,775	3,085	2,651
Restricted Cash and Cash Equivalents, Current	0	0
Asset Impairment Charges	0	0	0
Short-term Investments	65,493	57,103
Interest-bearing Deposits [Member]
Short-term Investments	$ 0
U.S. Plan [Member]
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	100.00%
Defined Contribution, Plan Employer Matching Contribution, Percent of Participants Contribution	50.00%
Foreign Pension Plan [Member] | VSUK Plan [Member]
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	100.00%
Defined Contribution Plan, Contributions, Percentage of Participants Base Pay	3.00%
Foreign Exchange Forward Contract Derivatives [Member]
Derivative, Loss on Derivative	$ 332	307
Employees and Consultants [Member]
Allocated Share-based Compensation Expense	$ 12,938	$ 7,794	$ 4,664
Israel, New Shekels
Investment Interest Rate	15.00%
Minimum [Member]
Short-term Investment, Maturity Period	90 days
Minimum [Member] | U.S. Plan [Member]
Defined Contribution Plan, Contributions, Percentage of Participants Base Pay	3.00%
Minimum [Member] | United States of America, Dollars
Investment Interest Rate	0.55%	0.30%
Maximum [Member]
Short-term Investment, Maturity Period	1 year
Maximum [Member] | U.S. Plan [Member]
Defined Contribution Plan, Employer Matching Contribution, Percent of Base Pay	3.00%
Defined Contribution Plan, Contributions, Percentage of Participants Base Pay	5.00%
Participant Matching Contribution Percentage of Base Remuneration	80.00%
Maximum [Member] | United States of America, Dollars
Investment Interest Rate	1.11%	1.00%