Note 7 - Fair Value Measurements - Assets and Liabilities at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Securities available for sale	$ 309,323	$ 304,282
Fair Value, Inputs, Level 1 [Member]
Securities available for sale	0	0
Fair Value, Inputs, Level 2 [Member]
Securities available for sale	309,323	304,282
Fair Value, Inputs, Level 3 [Member]
Securities available for sale	0	0
US Government Agencies Debt Securities [Member]
Securities available for sale	292,229	285,598
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Securities available for sale	0	0
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available for sale	292,229	285,598
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Securities available for sale	0	0
US States and Political Subdivisions Debt Securities [Member]
Securities available for sale	10,185	11,693
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Securities available for sale	0	0
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available for sale	10,185	11,693
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Securities available for sale	0	0
Collateralized Mortgage Backed Securities [Member]
Securities available for sale	711	930
Collateralized Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Securities available for sale	0	0
Collateralized Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available for sale	711	930
Collateralized Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Securities available for sale	0	0
Corporate Debt Securities [Member]
Securities available for sale	6,198	5,898
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Securities available for sale	0	0
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available for sale	6,198	5,898
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Securities available for sale	0	0
Other Securities [Member]
Securities available for sale		163
Other Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Securities available for sale	0	0
Other Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available for sale	0	163
Other Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Securities available for sale	$ 0	$ 0