Note 3 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans - Loans Past Due 90 Days or More and Impaired Loans (Details) - USD ($) $ in Thousands		3 Months Ended			12 Months Ended
		Mar. 31, 2018		Mar. 31, 2017	Dec. 31, 2017
Loans past due 90 days or more and still accruing		$ 52	[1]	$ 63	$ 51	[1]
Ratio of loans past due 90 days or more and still accruing to loans, net of unearned income and deferred fees and costs		0.01%		0.01%	0.01%
Accruing restructured loans		$ 7,890		$ 3,747	$ 5,134
Impaired loans:
Impaired loans with no valuation allowance		10,233	[2]	8,172	10,444	[2]
Impaired loans with a valuation allowance		1,448	[2]	894	1,480	[2]
Total impaired loans		11,681	[2]	9,066	11,924	[2]
Valuation allowance		(169)		(25)	(177)
Impaired loans, net of allowance		11,512		9,041	11,747
Average recorded investment in impaired loans(1)	[3],[4]	11,754		9,123	13,344	[5]
Interest income recognized on impaired loans, after designation as impaired		120		67	528	[5]
Amount of income recognized on a cash basis		$ 0		$ 0	$ 0

[1] Only classes with past-due or nonaccrual loans are shown.

[2] Recorded investment is net of charge-offs and interest paid while a loan is in nonaccrual status.

[3] Recorded investment is net of charge-offs and interest paid while a loan is in nonaccrual status.

[4] Recorded investment is net of charge-offs and interest paid while a loan is in nonaccrual status.

[5] Only classes with impaired loans are shown.