Note 10 - 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						35.00%
Effective Income Tax Rate Reconciliation, Percent, Total	1.90%	49.70%	4.70%	24.00%
Unrecognized Tax Benefits, Ending Balance	$ 4.1		$ 4.1			$ 4.1
Scenario, Forecast [Member]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent					21.00%