Note 7 - Fair Value Measurements - Assets and Liabilities at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Securities available for sale	$ 337,211	$ 331,387
Securities available for sale	337,211	331,387
Fair Value, Inputs, Level 1 [Member]
Securities available for sale	0
Securities available for sale		0
Fair Value, Inputs, Level 2 [Member]
Securities available for sale	337,211
Securities available for sale		331,387
Fair Value, Inputs, Level 3 [Member]
Securities available for sale	0
Securities available for sale		0
US Government Agencies Debt Securities [Member]
Securities available for sale	314,889
Securities available for sale	314,889	307,719
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Securities available for sale	0
Securities available for sale		0
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available for sale	314,889
Securities available for sale		307,719
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Securities available for sale	0
Securities available for sale		0
US States and Political Subdivisions Debt Securities [Member]
Securities available for sale	15,778
Securities available for sale	15,778	16,834
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Securities available for sale	0
Securities available for sale		0
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available for sale	15,778
Securities available for sale		16,834
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Securities available for sale	0
Securities available for sale		0
Collateralized Mortgage Backed Securities [Member]
Securities available for sale	608
Securities available for sale	608	659
Collateralized Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Securities available for sale	0
Securities available for sale		0
Collateralized Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available for sale	608
Securities available for sale		659
Collateralized Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Securities available for sale	0
Securities available for sale		0
Corporate Debt Securities [Member]
Securities available for sale	5,936
Securities available for sale	5,936	6,175
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Securities available for sale	0
Securities available for sale		0
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Securities available for sale	5,936
Securities available for sale		6,175
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Securities available for sale	$ 0
Securities available for sale		$ 0