Note 8 - Loans - Average Balance of Impaired Loans and Income Recognized on Impaired Loans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Average investment in impaired loans	$ 9,349	$ 10,185	$ 11,375
Interest income recognized on impaired loans on a cash basis	311	657	596
Residential Portfolio Segment [Member]
Average investment in impaired loans	969	877	2,270
Interest income recognized on impaired loans on a cash basis	67	46	90
Residential Portfolio Segment [Member] | Real Estate Loan [Member] | Senior Lien [Member]
Average investment in impaired loans	857	806	2,206
Interest income recognized on impaired loans on a cash basis	52	43	86
Residential Portfolio Segment [Member] | Real Estate Loan [Member] | Junior Lien [Member]
Average investment in impaired loans	112	71	64
Interest income recognized on impaired loans on a cash basis	15	3	4
Commercial Portfolio Segment [Member]
Average investment in impaired loans	8,354	9,287	9,105
Interest income recognized on impaired loans on a cash basis	243	610	506
Commercial Portfolio Segment [Member] | Real Estate Loan [Member]
Average investment in impaired loans	6,272	6,806	6,357
Interest income recognized on impaired loans on a cash basis	173	495	380
Commercial Portfolio Segment [Member] | Commercial and Industrial [Member]
Average investment in impaired loans	301	547	438
Interest income recognized on impaired loans on a cash basis	24	20	20
Commercial Portfolio Segment [Member] | Construction Loans [Member]
Average investment in impaired loans	0	0	40
Interest income recognized on impaired loans on a cash basis	0	0	0
Commercial Portfolio Segment [Member] | Loans Secured by Farmland [Member]
Average investment in impaired loans	1,379	1,409	1,459
Interest income recognized on impaired loans on a cash basis	45	94	103
Commercial Portfolio Segment [Member] | Multifamily 5 or More Residential [Member]
Average investment in impaired loans	392	511	790
Interest income recognized on impaired loans on a cash basis	0	0	0
Commercial Portfolio Segment [Member] | Agricultural Loans [Member]
Average investment in impaired loans	10	14	21
Interest income recognized on impaired loans on a cash basis	1	1	3
Consumer Portfolio Segment [Member]
Average investment in impaired loans	26	21	0
Interest income recognized on impaired loans on a cash basis	$ 1	$ 1	$ 0