Note 3 - Securities (Details Textual) $ in Thousands	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Number of Temporarily Impaired Securities	325
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 297,770
Continuous Unrealized Loss Position Aggregate Losses	$ 7,410
Number of Temporarily Impaired Securities Greater than Twelve Months	4
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	$ 2,475	$ 95,447
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	178	3,698
Available-for-sale Securities Pledged as Collateral	178,121	156,721
Federal Home Loan Bank of Atlanta [Member] | National Bank of Blacksburg [Member]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	493,620
Restricted Stock [Member]
Restricted Investments	$ 1,170	1,129
US Government Agencies Debt Securities [Member]
Number of Temporarily Impaired Securities	1
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 996
Continuous Unrealized Loss Position Aggregate Losses	4
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	996	84,959
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	$ 4	3,329
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	$ 1,316
Continuous Unrealized Loss Position Aggregate Losses	148
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	1,316	10,161
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	$ 148	306
Other Securities [Member]
Number of Temporarily Impaired Securities	1
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 26
Continuous Unrealized Loss Position Aggregate Losses	163
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	163	127
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	$ 26	$ 62