Note 7 - Fair Value Measurements - Assets and Liabilities at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Securities available for sale	$ 487,351	$ 435,263
Fair Value, Inputs, Level 2 [Member]
Securities available for sale	487,351	435,263
US Government Agencies Debt Securities [Member]
Securities available for sale	62,593	121,123
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available for sale	62,593	121,123
US States and Political Subdivisions Debt Securities [Member]
Securities available for sale	163,114	88,239
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available for sale	163,114	88,239
Collateralized Mortgage Backed Securities [Member]
Securities available for sale	258,324	221,783
Collateralized Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available for sale	258,324	221,783
Corporate Debt Securities [Member]
Securities available for sale	3,320	4,118
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available for sale	$ 3,320	$ 4,118