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Note 4 - Securities (Details Textual) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Securities, Held-to-maturity, Fair Value, Total	$ 0	$ 0		$ 119,790	$ 130,113
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment				$ 118,662	127,164
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, before Tax	$ 1,128	$ 1,128
Number of Temporarily Impaired Securities	391	391
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 351,924	$ 351,924
Continuous Unrealized Loss Position Aggregate Losses	$ 9,967	$ 9,967
Number of Temporarily Impaired Securities Greater than Twelve Months	246	246
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	$ 237,877	$ 237,877			229,675
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	6,873	6,873			4,750
Federal Home Loan Bank of Atlanta [Member] | National Bank of Blacksburg [Member]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	499,563	499,563
Restricted Stock [Member]
Restricted Investments	$ 1,221	$ 1,221			1,200
US Government Agencies Debt Securities [Member]
Debt Securities, Held-to-maturity, Fair Value, Total					4,101
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment					3,934
Number of Temporarily Impaired Securities	241	241
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 234,096	$ 234,096
Continuous Unrealized Loss Position Aggregate Losses	6,590	6,590
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	234,096	234,096			225,738
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	$ 6,590	$ 6,590			4,623
US States and Political Subdivisions Debt Securities [Member]
Debt Securities, Held-to-maturity, Fair Value, Total					124,795
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment					122,039
Number of Temporarily Impaired Securities	4	4
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 2,896	$ 2,896
Continuous Unrealized Loss Position Aggregate Losses	190	190
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	2,896	2,896			2,989
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	$ 190	$ 190			98
Corporate Debt Securities [Member]
Debt Securities, Held-to-maturity, Fair Value, Total					987
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment					982
Number of Temporarily Impaired Securities	1	1
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 885	$ 885
Continuous Unrealized Loss Position Aggregate Losses	93	93
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	885	885			948
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	$ 93	$ 93			$ 29

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This item represents the aggregate fair value of investments in debt and equity securities categorized as held-to-maturity and available for sale securities that have been in a continuous unrealized loss position.

+ References

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Amount of the collateral pledged in support of federal home loan bank advances outstanding.

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-Section 50
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Investments which are not defined as or included in marketable (debt, equity, or other) securities whose use is restricted in whole or in part, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet.

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