Note 1 - Summary of Significant Accounting Policies (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Operating Segments	1
Number of Reportable Segments	1
Trading Securities	$ 0	$ 0	$ 0
Other than Temporary Impairment Losses, Investments	0	0	0
Federal Home Loan Bank Stock and Federal Reserve Bank Stock	13,200	9,800
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums	$ 83	$ 301
Net Deferred Loan Costs, Percent of Average Loans Outstanding	0.01%	0.03%
Interest Receivable, Percent of Average Loans Outstanding	0.30%	0.30%
Unrecognized Tax Benefits	$ 0	$ 0	$ 0
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group, Mortgage	$ 0	$ 1,700
Mortgage Banking Revenues, Percent of Total Revenue	1.40%	(1.60%)	1.60%
Allowance for Loan and Lease Losses, Period Increase (Decrease)	$ 49
Servicing Asset at Fair Value, Amount	719	$ 731
Servicing Asset at Amortized Cost, Other than Temporary Impairments	0	0
Mortgage Loans Serviced for Others	198,000	202,000
Finite-Lived Intangible Assets, Net	$ 0	$ 0
Weighted Average Number Diluted Shares Outstanding Adjustment	0	0	0
Cash Reserve Deposit Required and Made	$ 17,300	$ 18,000
Stock Repurchased During Period, Shares	0	0
Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent	85.00%
Stock Issued During Period, Shares, Employee Stock Purchase Plans	5,367	6,631	6,329
Adjustments to Additional Paid in Capital, Share-based Compensation, Employee Stock Purchase Program, Requisite Service Period Recognition	$ 33	$ 32	$ 31
Reclassification of Tax Effects from AOCI
Retained Earnings [Member]
Adjustments to Additional Paid in Capital, Share-based Compensation, Employee Stock Purchase Program, Requisite Service Period Recognition
Reclassification of Tax Effects from AOCI	633
AOCI Attributable to Parent [Member]
Adjustments to Additional Paid in Capital, Share-based Compensation, Employee Stock Purchase Program, Requisite Service Period Recognition
Reclassification of Tax Effects from AOCI	(633)
ASU 2018-02 [Member] | Retained Earnings [Member]
Reclassification of Tax Effects from AOCI	633
ASU 2018-02 [Member] | AOCI Attributable to Parent [Member]
Reclassification of Tax Effects from AOCI	(633)
Customer Relationships [Member]
Finite-Lived Intangible Assets, Net	0	0	0
Core Deposits [Member]
Finite-Lived Intangible Assets, Net	$ 0	0	$ 0
Minimum [Member]
Finite-Lived Intangible Asset, Useful Life	7 years
Minimum [Member] | Furniture and Fixtures [Member]
Property, Plant and Equipment, Useful Life	2 years
Minimum [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment, Useful Life	10 years
Maximum [Member]
Finite-Lived Intangible Asset, Useful Life	10 years
Maximum [Member] | Furniture and Fixtures [Member]
Property, Plant and Equipment, Useful Life	7 years
Maximum [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment, Useful Life	40 years
Loans Receivable [Member]
Interest Receivable	$ 3,100	$ 2,800