Note 3 - Securities (Details Textual) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Number of Temporarily Impaired Securities	343
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 316,743
Continuous Unrealized Loss Position Aggregate Losses	$ 5,815
Number of Temporarily Impaired Securities Greater than Twelve Months	236
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	$ 229,675	$ 2,475
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	4,750	178
Available-for-sale Securities Pledged as Collateral	194,980	178,121
Federal Home Loan Bank of Atlanta [Member] | National Bank of Blacksburg [Member]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Available to be Pledged	494,666
Restricted Stock [Member]
Restricted Investments	$ 1,200	1,170
US Government Agencies Debt Securities [Member]
Number of Temporarily Impaired Securities	231
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 225,738
Continuous Unrealized Loss Position Aggregate Losses	4,623
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	225,738	996
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	$ 4,623	4
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	$ 2,989
Continuous Unrealized Loss Position Aggregate Losses	98
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	2,989	1,316
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	$ 98	148
Corporate Debt Securities [Member]
Number of Temporarily Impaired Securities	1
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value	$ 948
Continuous Unrealized Loss Position Aggregate Losses	29
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value	948	0
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses	29	0
Other Securities [Member]
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value		163
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses		$ 26
Available-for-sale Securities, Gross Realized Gains	4
Available-for-sale Securities Sold, Book Value	$ 189