Note 4 - Securities (Details Textual) $ in Thousands	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Number of Temporarily Impaired Securities Greater than Twelve Months	9
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	$ 8,738	$ 2,475
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	$ 83	178
Number of Temporarily Impaired Securities	261
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 255,263
Continuous Unrealized Loss Position Aggregate Losses	2,915
Federal Home Loan Bank of Atlanta [Member] | National Bank of Blacksburg [Member]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	480,370
Restricted Stock [Member]
Restricted Investments	1,200	1,170
US Government Agencies Debt Securities [Member]
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	7,949	996
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	$ 51	4
Number of Temporarily Impaired Securities	8
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 7,949
Continuous Unrealized Loss Position Aggregate Losses	51
US States and Political Subdivisions Debt Securities [Member]
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	789	1,316
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	$ 32	$ 148
Number of Temporarily Impaired Securities	1
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 789
Continuous Unrealized Loss Position Aggregate Losses	$ 32