Note 4 - Debt and Equity Securities - Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Due in one year or less, securities available-for-sale, amortized cost	$ 1,662
Due in one year or less, securities available-for-sale, estimated market value	1,663
Due after one year through five years, securities available-for-sale, amortized cost	42,049
Due after one year through five years, securities available-for-sale, estimated market value	42,662
Due after five years through ten years, securities available-for-sale, amortized cost	125,488
Due after five years through ten years, securities available-for-sale, estimated market value	127,325
Due after ten years, securities available-for-sale, amortized cost	395,910
Due after ten years, securities available-for-sale, estimated market value	403,353
Amortized cost	565,109	$ 420,207
Estimated market value	$ 575,003

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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