Note 5 - Investments in Affordable Housing and Alternative Energy Partnerships (Details Textual) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Investments in Affordable Housing and Alternative Energy Partnerships	$ 309,000	$ 308,700
Number of Limited Partnerships Deemed to be Variable Interest Entities	10
Increase in Total Assets and Liabilities from Consolidating Affordable Housing Equity Interest	$ 31,400	36,300
Alternative Energy Tax Credit Partnership Investments, Net	29,035	32,175
Alternative Energy Tax Credit Partnership Investments, Held in Escrow	9,300
Solar Tax Credits Expected to Be Realized	9,100
Asset Impairment Charges, Total	7,836	848	$ (289)
Affordable Housing Limited Partnerships [Member]
Asset Impairment Charges, Total	1,400	$ 2,100
Affordable Housing Limited Partnerships [Member] | Domestic Tax Authority [Member]
Remaining and Future Available Tax Credit	218,600
Affordable Housing Limited Partnerships [Member] | State and Local Jurisdiction [Member]
Remaining and Future Available Tax Credit	$ 3,100