Note 4 - Securities (Details Textual) $ in Thousands	3 Months Ended
	Jun. 30, 2018 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)
Debt Securities, Held-to-maturity, Fair Value, Total	$ 119,790			$ 124,549
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment	118,662			122,905
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, before Tax	$ 1,128
Number of Temporarily Impaired Securities		266
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value		$ 252,300
Continuous Unrealized Loss Position Aggregate Losses		$ 3,208
Number of Temporarily Impaired Securities Greater than Twelve Months		264
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value		$ 250,803	$ 281,626
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses		3,123	7,365
Federal Home Loan Bank of Atlanta [Member] | National Bank of Blacksburg [Member]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged		522,582
Restricted Stock [Member]
Restricted Investments		$ 1,220	1,220
US Government Agencies Debt Securities [Member]
Debt Securities, Held-to-maturity, Fair Value, Total				4,032
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment				3,934
Number of Temporarily Impaired Securities		239
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value		$ 235,610
Continuous Unrealized Loss Position Aggregate Losses		2,859
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value		235,610	259,992
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses		$ 2,859	6,450
US States and Political Subdivisions Debt Securities [Member]
Debt Securities, Held-to-maturity, Fair Value, Total				119,326
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment				117,791
Number of Temporarily Impaired Securities		22
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value		$ 12,300
Continuous Unrealized Loss Position Aggregate Losses		164
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value		12,300	20,758
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses		$ 164	814
Corporate Debt Securities [Member]
Debt Securities, Held-to-maturity, Fair Value, Total				977
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment				$ 984
Number of Temporarily Impaired Securities		3
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value		$ 2,893
Continuous Unrealized Loss Position Aggregate Losses		100
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value		2,893	876
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses		$ 100	$ 101