Note 4 - Securities (Details Textual) $ in Thousands	3 Months Ended	9 Months Ended
	Jun. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Securities, Held-to-maturity, Fair Value, Total		$ 0		$ 119,790	$ 130,113
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment				$ 118,662	127,164
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, before Tax	$ 1,128	$ 1,128
Number of Temporarily Impaired Securities		413
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value		$ 366,365
Continuous Unrealized Loss Position Aggregate Losses		$ 11,720
Number of Temporarily Impaired Securities Greater than Twelve Months		263
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value		$ 250,876			229,675
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses		7,693			4,750
Federal Home Loan Bank of Atlanta [Member] | National Bank of Blacksburg [Member]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged		503,690
Restricted Stock [Member]
Restricted Investments		$ 2,708			1,200
US Government Agencies Debt Securities [Member]
Debt Securities, Held-to-maturity, Fair Value, Total					4,101
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment					3,934
Number of Temporarily Impaired Securities		252
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value		$ 244,485
Continuous Unrealized Loss Position Aggregate Losses		7,181
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value		244,485			225,738
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses		$ 7,181			4,623
US States and Political Subdivisions Debt Securities [Member]
Debt Securities, Held-to-maturity, Fair Value, Total					124,795
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment					122,039
Number of Temporarily Impaired Securities		10
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value		$ 5,514
Continuous Unrealized Loss Position Aggregate Losses		411
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value		5,514			2,989
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses		$ 411			98
Corporate Debt Securities [Member]
Debt Securities, Held-to-maturity, Fair Value, Total					987
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment					982
Number of Temporarily Impaired Securities		1
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value		$ 877
Continuous Unrealized Loss Position Aggregate Losses		101
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value		877			948
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses		$ 101			$ 29