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		9 Months Ended			12 Months Ended
		Sep. 30, 2018		Sep. 30, 2017	Dec. 31, 2017
Loans past due 90 days or more and still accruing		$ 63	[1]	$ 250	$ 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%		0.04%	0.01%
Accruing restructured loans		$ 7,843		$ 4,815	$ 5,134
Impaired loans:
Impaired loans with no valuation allowance		10,530	[2]	10,522	10,444	[2]
Impaired loans with a valuation allowance		1,404	[2]	1,637	1,480	[2]
Total impaired loans		11,934	[2]	12,159	11,924	[2]
Valuation allowance		(156)		(186)	(177)
Impaired loans, net of allowance		11,778		11,973	11,747
Average recorded investment in impaired loans(1)	[3],[4]	12,684		12,541	13,344	[5]
Interest income recognized on impaired loans, after designation as impaired		414		387	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.