Note 4 - Realized Gain (Loss) on Investments (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Gross realized gains	$ 169,000	$ 25,000	$ 581,000
Gross realized losses	(195,000)	(22,000)	(558,000)
Other-than-temporary impairment	0	0	0
Total net (losses) gains on AFS debt securities	$ (26,000)	$ 3,000	$ 23,000