Note 8 - Income Taxes (Details Textual) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	26.00%	28.00%		35.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Research, Amount		$ 13.6
Unrecognized Tax Benefits, Ending Balance	$ 46.1
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	36.4
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued, Total	$ 4.1
Scenario, Forecast [Member]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			21.00%