Note 4 - Securities (Details Textual) $ in Thousands	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Number of Temporarily Impaired Securities	314
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 292,607
Continuous Unrealized Loss Position Aggregate Losses	$ 5,343
Number of Temporarily Impaired Securities Greater than Twelve Months	5
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	$ 3,564	$ 2,475
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	88	178
Federal Home Loan Bank of Atlanta [Member] | National Bank of Blacksburg [Member]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	478,903
Restricted Stock [Member]
Restricted Investments	$ 1,200	1,170
US Government Agencies Debt Securities [Member]
Number of Temporarily Impaired Securities	2
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 1,995
Continuous Unrealized Loss Position Aggregate Losses	4
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	1,995	996
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	$ 4	4
US States and Political Subdivisions Debt Securities [Member]
Number of Temporarily Impaired Securities	2
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 1,389
Continuous Unrealized Loss Position Aggregate Losses	75
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	1,389	1,316
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	$ 75	148
Other Securities [Member]
Number of Temporarily Impaired Securities	1
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 180
Continuous Unrealized Loss Position Aggregate Losses	9
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	180	163
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	$ 9	$ 26