Note 15 - Fair Value Measurements - Assets and Liabilities at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Securities available-for-sale, fair values	$ 435,263	$ 425,010
Fair Value, Inputs, Level 1 [Member]
Securities available-for-sale, fair values	0	0
Fair Value, Inputs, Level 2 [Member]
Securities available-for-sale, fair values	435,263	425,010
Fair Value, Inputs, Level 3 [Member]
Securities available-for-sale, fair values	0	0
US Government Agencies Debt Securities [Member]
Securities available-for-sale, fair values	121,123	300,047
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Securities available-for-sale, fair values	0	0
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available-for-sale, fair values	121,123	300,047
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Securities available-for-sale, fair values	0	0
US States and Political Subdivisions Debt Securities [Member]
Securities available-for-sale, fair values	88,239	118,616
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Securities available-for-sale, fair values	0	0
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available-for-sale, fair values	88,239	118,616
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Securities available-for-sale, fair values	0	0
Collateralized Mortgage Backed Securities [Member]
Securities available-for-sale, fair values	221,783	628
Collateralized Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Securities available-for-sale, fair values	0	0
Collateralized Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available-for-sale, fair values	221,783	628
Collateralized Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Securities available-for-sale, fair values	0	0
Corporate Debt Securities [Member]
Securities available-for-sale, fair values	4,118	5,719
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Securities available-for-sale, fair values	0	0
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available-for-sale, fair values	4,118	5,719
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Securities available-for-sale, fair values	$ 0	$ 0