Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans - Loans Past Due 90 Days or More and Impaired Loans (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Loans past due 90 days or more and still accruing		$ 51		$ 63
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%
Accruing restructured loans		$ 5,134		$ 3,769
Impaired loans:
Impaired loans with no valuation allowance	[1]	10,444		8,269
Impaired loans with a valuation allowance	[1]	1,480		904
Total impaired loans	[1]	11,924		9,173
Valuation allowance		(177)		(26)
Impaired loans, net of allowance		11,747		9,147
Average recorded investment in impaired loans	[3]	13,344	[2]	11,585	[2]	$ 17,297
Income recognized on impaired loans, after designation as impaired		528		553		$ 769
Amount of income recognized on a cash basis		$ 0		$ 0

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

[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.