Note 4 - Securities (Details Textual) $ in Thousands	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Number of Temporarily Impaired Securities	283
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 273,273
Continuous Unrealized Loss Position Aggregate Losses	$ 3,303
Number of Temporarily Impaired Securities Greater than Twelve Months	91
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	$ 89,029	$ 2,475
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	1,553	178
Federal Home Loan Bank of Atlanta [Member] | National Bank of Blacksburg [Member]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	490,534
Restricted Stock [Member]
Restricted Investments	$ 1,200	1,170
US Government Agencies Debt Securities [Member]
Number of Temporarily Impaired Securities	89
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 87,469
Continuous Unrealized Loss Position Aggregate Losses	1,513
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	87,469	996
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	$ 1,513	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	$ 1,560
Continuous Unrealized Loss Position Aggregate Losses	40
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	1,560	1,316
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	$ 40	$ 148