Note 1 - Summary of Significant Accounting Policies (Details Textual) - USD ($) shares in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2019	Jan. 01, 2019
Debt Securities, Held-to-maturity, Transfer, Amount		$ 119,790,000
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, before Tax	$ 1,128,000	$ 1,128,000
Number of Days of Delinquent Loans Nonaccrual of Interest		90 days
Minimum Number of Days of Discontinued Loan Payments for Modified Loan to be Placed on Nonaccrual		90 days
Number of Days Past Due		30 days
Threshold for Designation to Impaired Status		$ 250,000
Minimum Number of Days Consumer Loans are Past Due for Losses to be Charged Off		120 days
Maximum Number of Days Consumer Loans are Past Due for Losses to be Charged Off		180 days
Number of Days Loans Secured by Residential Or Commercial Real Estate Are Past Due for Losses to Be Carged Off		180 days
Period of Time Between Issuance of Loan Commitment and Closing and Sale of Loans Lower Range		30 days
Period of Time Between Issuance of Loan Commitments and Closing and Sale of Loans Upper Range		60 days
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%	35.00%	35.00%
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability			$ 1,560,000
Reclassification of Stranded Tax Effects from Change in Tax Rate
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount		0	0
Advertising Expense		$ 106,000	$ 148,000	122,000
Assets, Total		1,256,032,000	1,256,757,000
Liabilities, Total		$ 1,065,794,000	1,071,861,000
Scenario, Forecast [Member]
Operating Lease, Right-of-Use Asset					$ 1,529,000
Operating Lease, Liability, Total					$ 1,529,000
Accounting Standards Update 2014-09 [Member] | Credit Card Processing Expense Reclassified to Net Against Credit Card Fee Income [Member]
Prior Period Reclassification Adjustment			2,743,000	$ 2,817,000
Accounting Standards Update 2016-02 [Member] | Subsequent Event [Member]
Assets, Total						$ 684,000
Liabilities, Total						$ 684,000
ASU 2018-02 [Member]
Reclassification of Stranded Tax Effects from Change in Tax Rate			$ 1,718,000
National Bank of Blacksburg [Member]
Noncontrolling Interest, Ownership Percentage by Parent		100.00%
Minimum [Member]
Finite-Lived Intangible Asset, Useful Life		10 years
Maximum [Member]
Finite-Lived Intangible Asset, Useful Life		12 years
Building [Member]
Property, Plant and Equipment, Useful Life		40 years
Furniture and Fixtures [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life		3 years
Furniture and Fixtures [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life		10 years
Computer Equipment [Member]
Property, Plant and Equipment, Useful Life		3 years