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Note 3 - Investment Securities (Details Textual)
1 Months Ended 12 Months Ended
Sep. 30, 2018
USD ($)
shares
Dec. 31, 2021
USD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Oct. 31, 2021
USD ($)
Debt Securities, Available-for-sale, Amortized Cost, Total   $ 445,220,000 $ 274,709,000    
Number of Holdings of Securities with an Aggregate Book Value Greater than 10% of Stockholders' Equity   0 0    
Available-for-Sale, Securities in Unrealized Loss Positions, Depreciation from Amortized Cost Percentage   1.40%      
Other-than-temporary Impairment Loss, Debt Securities, Available-for-sale, Total   $ 0      
Debt Securities, Available-for-sale, Realized Gain   12,000   $ 22,000  
Debt Securities, Available-for-sale, Realized Loss   5,000   156,000  
Proceeds from Sale of Debt Securities, Available-for-sale   1,798,000 $ 0 21,725,000  
Investment, Number of Shares Acquired (in shares) | shares 90,000        
Investment Ownership Percentage 5.00%        
Payments to Acquire Other Investments $ 1,900,000        
Equity Securities, FV-NI, Unrealized Gain   328,000   $ 32,000  
Equity Securities, FV-NI, Unrealized Loss     194,000    
Equity Securities, FV-NI, Current   1,900,000 $ 1,600,000    
Limited Partnership Interest, Technology Fund [Member]          
Equity Securities without Readily Determinable Fair Value, Impairment Loss, Annual Amount   0      
Limited Partnership Interest, Technology Fund [Member] | Other Assets [Member]          
Equity Securities without Readily Determinable Fair Value, Amount         $ 1,000,000.0
Limited Partnership Interest, Technology Fund [Member] | Other Liabilities [Member]          
Investment Company, Financial Commitment to Investee, Future Amount         $ 880,000
Collateral Pledged [Member]          
Debt Securities, Available-for-sale, Amortized Cost, Total   101,700,000      
Debt Securities, Available-for-sale, Restricted   $ 100,900,000