Note 6 - Investments in Affordable Housing and Alternative Energy Partnerships (Details Textual) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Investments in Affordable Housing and Alternative Energy Partnerships	$ 272,900	$ 282,700	$ 272,900
Number Of Limited Partnerships In Housing Investments		8
Increase in Total Assets and Liabilities from Consolidating Affordable Housing Equity Interest		$ 23,800	23,600
Alternative Energy Tax Credit Partnership Investments, Net	12,759	23,688	12,759
Alternative Energy Tax Credit Partnership Investments, Held in Escrow		7,800
Solar Tax Credits Expected to Be Realized		8,000
Asset Impairment Charges, Total		(289)	874	$ 1,307
Low income Housing Tax Credit Investments, Pretax Write-down	$ 2,600		$ 2,600
Affordable Housing Limited Partnerships [Member]
Asset Impairment Charges, Total		4,500
Affordable Housing Limited Partnerships [Member] | Domestic Tax Authority [Member]
Remaining and Future Available Tax Credit		200,500
Affordable Housing Limited Partnerships [Member] | State and Local Jurisdiction [Member]
Remaining and Future Available Tax Credit		$ 1,800