Note 8 - Income Taxes (Details Textual) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	26.00%	37.40%	26.00%	32.90%		35.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Research, Amount				$ 13.6
Unrecognized Tax Benefits, Ending Balance	$ 46.9		$ 46.9
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	37.1		37.1
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued, Total	$ 4.7		$ 4.7
Scenario, Forecast [Member]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent					21.00%