Note 7 - Fair Value Measurements - Fair Value of Financial Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available-for-sale securities	$ 49,571	$ 55,344
US Government Agencies Debt Securities [Member]
Available-for-sale securities	12,406	13,435
US Government Agencies Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities	12,406	13,435
US Government Agencies Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities	12,406	13,435
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Available-for-sale securities	21,425	23,981
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities	21,425
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities	21,425	23,981
Corporate Debt Securities [Member]
Available-for-sale securities	6,618	7,067
Corporate Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities	6,618	7,067
Corporate Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities	6,618	7,067
US States and Political Subdivisions Debt Securities [Member]
Available-for-sale securities	9,122	10,861
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities	9,122	10,861
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities	$ 9,122	$ 10,861