Note 9 - Fair Value Measurement - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale	$ 159,247,000	$ 159,320,000
Fair Value, Recurring [Member]
Debt Securities, Available-for-sale	159,247	159,320
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Debt Securities, Available-for-sale	159,247	159,320
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale	125,895,000	125,678,000
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Fair Value, Recurring [Member]
Debt Securities, Available-for-sale	125,895	125,678
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Debt Securities, Available-for-sale	125,895	125,678
US States and Political Subdivisions Debt Securities [Member]
Debt Securities, Available-for-sale	33,352,000	33,642,000
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Recurring [Member]
Debt Securities, Available-for-sale	33,352	33,642
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Debt Securities, Available-for-sale	$ 33,352	$ 33,642