Note 3 - Securities (Details Textual)	3 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2018 USD ($)
Debt Securities, Held-to-maturity, Fair Value, Total		$ 0	$ 130,113,000		$ 119,790,000
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment			127,164,000		$ 118,662,000
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, before Tax	$ 1,128,000	$ 1,128,000
Number of Temporarily Impaired Securities		359
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value		$ 321,080,000
Continuous Unrealized Loss Position Aggregate Losses		$ 8,127,000
Number of Temporarily Impaired Securities Greater than Twelve Months		309
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value		$ 281,626,000	229,675,000
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses		7,365,000	4,750,000
Debt Securities, Available-for-sale, Restricted		196,062,000	194,980,000
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		513,578,000
Restricted Stock [Member]
Restricted Investments		$ 1,220,000	1,200,000
US Government Agencies Debt Securities [Member]
Debt Securities, Held-to-maturity, Fair Value, Total			4,101,000
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment			3,934,000
Number of Temporarily Impaired Securities		267
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value		$ 259,992,000
Continuous Unrealized Loss Position Aggregate Losses		6,450,000
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value		259,992,000	225,738,000
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses		$ 6,450,000	4,623,000
US States and Political Subdivisions Debt Securities [Member]
Debt Securities, Held-to-maturity, Fair Value, Total			124,795,000
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment			122,039,000
Number of Temporarily Impaired Securities		41
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value		$ 20,758,000
Continuous Unrealized Loss Position Aggregate Losses		814,000
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value		20,758,000	2,989,000
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses		$ 814,000	98,000
Corporate Debt Securities [Member]
Debt Securities, Held-to-maturity, Fair Value, Total			987,000
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment			982,000
Number of Temporarily Impaired Securities		1
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value		$ 876,000
Continuous Unrealized Loss Position Aggregate Losses		101,000
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value		876,000	948,000
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses		101,000	29,000
Other Securities [Member]
Available-for-sale Securities, Gross Realized Gains		$ 16,000	4,000
Available-for-sale Securities Sold, Book Value			$ 189,000