Note 2 - Significant Accounting Policies (Details Textual) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 01, 2017
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities		$ 0
Restricted Cash and Cash Equivalents, Current		0	$ 0
Restricted Cash and Cash Equivalents, Noncurrent		547	488	$ 467
Asset Impairment Charges		$ 0	$ 0	$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross		0	135,000	191,200
Fair Value Assumptions, Expected Dividend Rate			0.00%	0.00%
Fair Value Assumptions, Expected Volatility Rate			62.10%	65.00%
Fair Value Assumptions, Risk Free Interest Rate			1.42%
Fair Value Assumptions, Expected Term			6 years 91 days	6 years 91 days
Allocated Share-based Compensation Expense		$ 19,835	$ 12,938	$ 7,794
Minimum Tax Percentage Attained Likely to be Realized upon Ultimate Settlement		50.00%
Severance and Retirement Costs		$ 4,801	3,775	3,085
Revenues		217,364	164,456	127,210
Deferred Tax Assets, Valuation Allowance		26,122	28,500
Other Noncurrent Liabilities [Member]
Restructuring Reserve		$ 1,781	1,664
Foreign Plan [Member] | VSUK Plan [Member]
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent		8.33%
UNITED KINGDOM | 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%
Deferred Salary Arrangement [Member] | UNITED STATES
Defined Contribution Plan, Employer Matching Contribution, Percent of Match		100.00%
Defined Contribution, Plan Employer Matching Contribution, Percent of Participants Contribution		50.00%
Foreign Exchange Contract [Member]
Derivative, Gain (Loss) on Derivative, Net		$ 2,649	332
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net		0	0
Accounting Standards Update 2016-09 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption
Deferred Tax Assets, Operating Loss Carryforwards, Domestic					$ 7,131
Deferred Tax Assets, Operating Loss Carryforwards, State and Local					718
Deferred Tax Assets, Valuation Allowance					7,849
Accounting Standards Update 2016-09 [Member] | Domestic Tax Authority [Member]
Deferred Tax Assets, Valuation Allowance					7,131
Accounting Standards Update 2016-09 [Member] | State and Local Jurisdiction [Member]
Deferred Tax Assets, Valuation Allowance					$ 718
Accounting Standards Update 2014-09 [Member] | Scenario, Forecast [Member] | As of January 1, 2016 [Member]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets	$ 5,583
Accounting Standards Update 2014-09 [Member] | Scenario, Forecast [Member] | Restatement Adjustments [Member] | Year Ended December 31, 2017 [Member]
Revenues	(1,974)
Accounting Standards Update 2014-09 [Member] | Scenario, Forecast [Member] | Restatement Adjustments [Member] | Year Ended December 31, 2016 [Member]
Revenues	1,407
Accounting Standards Update 2014-09 [Member] | Scenario, Forecast [Member] | Restatement Adjustments [Member] | As of December 31, 2017 [Member]
Prepaid Expense	9,865
Deferred Revenue	$ (97)
Retained Earnings [Member] | Accounting Standards Update 2016-09 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption			(2,616)
Employees and Consultants [Member]
Allocated Share-based Compensation Expense		$ 19,835	$ 12,938	$ 7,794
Israel, New Shekels
Investment Interest Rate		0.03%	0.15%
Minimum [Member]
Fair Value Assumptions, Risk Free Interest Rate				1.94%
Minimum [Member] | Deferred Salary Arrangement [Member] | UNITED STATES
Defined Contribution Plan, Contributions, Percentage of Participants Base Pay		3.00%
Minimum [Member] | United States of America, Dollars
Investment Interest Rate		0.60%	0.55%
Maximum [Member]
Fair Value Assumptions, Risk Free Interest Rate				2.00%
Maximum [Member] | Deferred Salary Arrangement [Member] | UNITED STATES
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		0.135%	0.111%