Note 6 - Fair Value Measurements - Assets and Liabilities at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Securities available for sale	$ 571,359	$ 546,742
Fair Value, Inputs, Level 2 [Member]
Securities available for sale	571,539	546,742
US Government Agencies Debt Securities [Member]
Securities available for sale	130,225	91,163
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available for sale	130,225	91,163
US States and Political Subdivisions Debt Securities [Member]
Securities available for sale	199,185	203,961
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available for sale	199,185	203,961
Collateralized Mortgage Backed Securities [Member]
Securities available for sale	239,796	249,175
Collateralized Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available for sale	239,796	249,175
Corporate Debt Securities [Member]
Securities available for sale	2,153	2,443
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available for sale	$ 2,153	$ 2,443