Note 6 - Investments in Affordable Housing and Alternative Energy Partnerships (Details Textual) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Number Of Limited Partnerships In Housing Investments	8	8
Increase in Total Assets and Liabilities from Consolidating Affordable Housing Equity Interest		$ 23.6	$ 23.7
Low income Housing Tax Credit Investments, Pretax Write-down	$ 2.6	2.6
Affordable Housing Limited Partnerships [Member] | Domestic Tax Authority [Member]
Remaining and Future Available Tax Credit	190.6	190.6
Affordable Housing Limited Partnerships [Member] | State and Local Jurisdiction [Member]
Remaining and Future Available Tax Credit	$ 2.7	$ 2.7