Note 7 - Income Taxes (Details Textual) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%
Income Taxes Receivable	$ 3.1	$ 60.9
Unrecognized Tax Benefits, Ending Balance	66.1	50.6	$ 52.2	$ 45.3
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	57.1	41.8
Income Tax Examination, Penalties and Interest Accrued, Total	2.9	3.2	$ 2.4
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued, Total	$ 5.6	$ 4.8
Domestic Tax Authority [Member] | Internal Revenue Service (IRS) [Member]
Open Tax Year	2017 2018 2019 2020
State and Local Jurisdiction [Member]
Open Tax Year	2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020