Document And Entity Information - shares	9 Months Ended
	Sep. 30, 2020	Nov. 06, 2020
Document Information [Line Items]
Entity Central Index Key	0000796534
Entity Registrant Name	NATIONAL BANKSHARES INC
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	Q3
Document Fiscal Year Focus	2020
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2020
Document Transition Report	false
Entity File Number	0-15204
Entity Incorporation, State or Country Code	VA
Entity Tax Identification Number	54-1375874
Entity Address, Address Line One	101 Hubbard Street
Entity Address, City or Town	Blacksburg
Entity Address, State or Province	VA
Entity Address, Postal Zip Code	24062-9002
City Area Code	540
Local Phone Number	951-6300
Title of 12(b) Security	Common Stock, par value $1.25 per share
Trading Symbol	NKSH
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		6,478,997