Note 6 - Investments in Affordable Housing and Alternative Energy Partnerships (Details Textual) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Affordable Housing Investments	$ 236.8	$ 172.7
Number Of Limited Partnerships In Housing Investments	7
Increase in Total Assets and Liabilities from Consolidating Affordable Housing Equity Interest	$ 23.7	24.3
Other Borrowings from Consolidating Affordable Housing Equity Interest	17.7	18.6
Other Liabilities from Affordablehousing Unfunded Commitments	115.0	85.8
Affordable Housing Investments, Scheduled Funding for Unfunded Commitments in Next 12 months	52.7
Affordable Housing Investments, Scheduled Funding for Unfunded Commitments in Year Two	33.4
Affordable Housing Investments, Scheduled Funding for Unfunded Commitments in Year Three	22.7
Affordable Housing Investments, Scheduled Funding for Unfunded Commitments in Year Four	2.6
Affordable Housing Investments, Scheduled Funding for Unfunded Commitments in Year Five	0.6
Affordable Housing Investments, Scheduled Funding for Unfunded Commitments Thereafter	3.0
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Amount	13.4	10.1	$ 10.2
Alternative Energy Tax Credit Partnership Investments, Net	14.3	10.5
Energy Tax Credits Used, Amount	24.5	$ 20.9
Affordable Housing Limited Partnerships [Member] | Domestic Tax Authority [Member]
Remaining and Future Available Tax Credit	160.2
Affordable Housing Limited Partnerships [Member] | State and Local Jurisdiction [Member]
Remaining and Future Available Tax Credit	$ 3.3