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Note 4 - Securities (Details Textual) $ in Thousands	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Number of Temporarily Impaired Securities	377
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 341,058
Continuous Unrealized Loss Position Aggregate Losses	$ 9,260
Number of Temporarily Impaired Securities Greater than Twelve Months	238
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	$ 230,059	$ 229,675
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	6,369	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	488,193
Restricted Stock [Member]
Restricted Investments	$ 1,221	1,200
US Government Agencies Debt Securities [Member]
Number of Temporarily Impaired Securities	233
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 226,261
Continuous Unrealized Loss Position Aggregate Losses	6,103
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	226,261	225,738
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	$ 6,103	4,623
US States and Political Subdivisions Debt Securities [Member]
Number of Temporarily Impaired Securities	4
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 2,889
Continuous Unrealized Loss Position Aggregate Losses	198
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	2,889	2,989
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	$ 198	98
Corporate Debt Securities [Member]
Number of Temporarily Impaired Securities	1
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 909
Continuous Unrealized Loss Position Aggregate Losses	68
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	909	948
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	$ 68	$ 29

X

- Definition

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

No definition available.

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- Definition

Amount of the excess of amortized cost basis over fair value of securities that have been in a loss position for twelve months or longer for securities.

+ References

No definition available.

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Amount of the excess of amortized cost basis over fair value of securities that are in a loss position.

+ References

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This item represents the aggregate fair value of investments in debt and equity securities that have been in a continuous unrealized loss position for twelve months or longer.

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This item contains disclosure of the number of investment positions determined to be temporarily impaired in a continuous loss position for twelve months or more.

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822

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- Definition

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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