Note 7 - Fair Value Measurement - Fair Value of Financial Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Available-for-sale securities	$ 47,915	$ 55,344	[1]
US Government Agencies Debt Securities [Member]
Available-for-sale securities	11,770	13,435
US Government Agencies Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities	11,770	13,435
US Government Agencies Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities	11,770	13,435
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Available-for-sale securities	20,434	23,981
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities	20,434
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	20,434	23,981
Corporate Debt Securities [Member]
Available-for-sale securities	6,610	7,067
Corporate Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities	6,610	7,067
Corporate Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities	6,610	7,067
US States and Political Subdivisions Debt Securities [Member]
Available-for-sale securities	9,101	10,861
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities	9,101	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	$ 9,101	$ 10,861

[1] Derived from consolidated audited financial statements.