Note 5 - Investments in Affordable Housing and Alternative Energy Partnerships (Details Textual) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Investments in Affordable Housing and Alternative Energy Partnerships	$ 308,700	$ 282,700
Number of Limited Partnerships Deemed to be Variable Interest Entities	10
Increase in Total Assets and Liabilities from Consolidating Affordable Housing Equity Interest	$ 36,300	23,800
Alternative Energy Tax Credit Partnership Investments, Net	32,175	23,688
Alternative Energy Tax Credit Partnership Investments, Held in Escrow	18,900
Solar Tax Credits Expected to Be Realized	14,900
Asset Impairment Charges, Total	848	(289)	$ 874
Affordable Housing Limited Partnerships [Member]
Asset Impairment Charges, Total	2,100	$ 4,500
Affordable Housing Limited Partnerships [Member] | Domestic Tax Authority [Member]
Remaining and Future Available Tax Credit	207,600
Affordable Housing Limited Partnerships [Member] | State and Local Jurisdiction [Member]
Remaining and Future Available Tax Credit	$ 1,500